佢喺大分縣出世，喺神奈川縣長大，受比佢大3歲嘅哥哥影響，由幼稚園開始踢波。他由小學二年級開始正式接受訓練，後黎通過左川崎前鋒既選拔，由小學三年級開始成為川崎前鋒既U-12成員。喺U-12巴3時候，三笘同大佢一年既田中碧，三好康兒，板倉滉等一齊接受訓練，而且喺5年級，6年級既時候(隊長) 連續兩年晉級去全國少年足球大會，但分別喺八強同埋四強既時候輸俾名名古屋鯨魚U-12。
He was born in Oita Prefecture and grew up in Kanagawa Prefecture, where he was influenced by his older brother, who was 3 years older than him, and started playing football in kindergarten. He began his formal training in the second grade of elementary school, and later became a member of the U-12 of Kawasaki Frontale from the third grade through the selection of Left Kawasaki Frontale. In the U-12 Ba3 time, Mikasa trained with Bi Tanaka, Yasuko Miyoshi, Itakura and others who were a year older than him, and in the 5th and 6th grades (captain) he advanced to the National Youth Soccer Championship for two consecutive years, but lost to the Nagoya Whales U-12 in the quarterfinals and semifinals respectively.