The glutinous dish is a part of many people's celebration of New Year.
According to local reports, 18 people were taken to hospital in Tokyo after suffocating on the snack. Three of those died, an official from the fire department said.
The Yomiuri Shimbun newspaper said the death toll from rice cake accidents reached nine by Friday across Japan.
Another 13 people were in a serious condition.
The fire department is advising people - especially the very young and elderly - to cut mochi into small pieces before consuming.
Several people die each year in Japan after choking on rice cakes.
New Year is one of Japan's biggest holidays.
Families traditionally celebrate by cooking a vegetable broth called "ozouni" and then putting the rice cakes into the soup.
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Mohammed Ali, of Prescot Road, Liverpool, is accused of attempting to possess a chemical weapon.
The father of two, 31, has denied the charge at the Old Bailey in London.
Using an online alias, Ali allegedly tried to order 500mg of the deadly toxin, the jury was told, which would have been enough to kill 1,400 people.
As "Weirdos 0000", Ali contacted a man on the black market and placed an order for the ricin, the court heard.
He went on to negotiate the deal in encrypted exchanges unaware the United States-based 'dealer' was actually an FBI agent who alerted the British authorities.
On 10 February, he took delivery of a toy car with "special batteries" at the home he shared with his wife and two young sons.
But instead of ricin, the five concealed packets contained a harmless powder and Ali was arrested.
The court has been told that the Bolton-born computer software programmer has displayed many traits of Asperger's syndrome.
Under cross-examination, defence clinical psychologist Alison Beck said: "I think that so far as I understand it, Mr Ali was motivated with pushing the boundaries of what was possible with the technology.
"The relevance of the dark net was to procure ricin and that idea was implanted in his brain having watched the series Breaking Bad."
Ali denies a charge of attempting to possess a chemical weapon between 10 January and 12 February.
The trial continues.

The Age UK review identified more than three million people aged over 65 with a care need, but found just two-thirds of them were actually getting help.
The charity warned that the lack of support for the ageing population was risking their health.
But ministers said steps were being taken to provide more help.
The review used official data and existing research to identify how many people were struggling with everyday tasks and how many were getting help.
There are 10 million people over the age of 65 in England, the review said, and more than 3 million struggle with tasks such as washing, dressing, eating and going to the toilet.
Just over one million pay for care or rely on family and friends with another 850,000 supported by their local councils.
But that leaves another one million who have to fend for themselves.
The report also warned that community NHS services and GPs were struggling to meet demand from the ageing population.
And Age UK warned there were signs this was affecting the health of older people.
It cited an 88% rise in hospital admissions for urinary tract infections among the over-75s between 2005-06 and 2013-14 to 4,173 per 100,000.
Meanwhile, admissions for pneumonia among the over-60s have more than doubled over the same period to 2,621 per 100,000.
Age UK charity director Caroline Abrahams said it was a "destructive vicious circle" where the lack of support was worsening the health of the most vulnerable older people in society.
"If an older person asked us today how confident we were that their health and care needs will be met well in the future we would be whistling in the dark if we gave a wholly reassuring answer."
But a Department of Health spokeswoman said "significant action" was being taken to improve the support given to older people.
She pointed out that all over-75s should now have a named GP to co-ordinate their care, while a Â£5.3bn pot of money - mainly from NHS funds - has been set aside this year for joint projects between councils and health services.
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Nine people have choked to death in Japan after eating a traditional rice cake, known as "mochi".
[NEXT_CONCEPT]
A man tried to buy ricin from the "dark web" after the idea was "implanted in his brain" from watching the Breaking Bad television series, a court heard.
[NEXT_CONCEPT]
A million older people in England struggling with everyday tasks, such as washing and dressing, are being left to fend for themselves, campaigners say.