def _check_notebook_extension(notebook_path):
    """
    Check the path extension of the notebook.

    Parameters
    ----------
    notebook_path : str
        Path of target notebook.

    Raises
    ------
    IOError
        If the extension is invalid.
    """
    extension_str = notebook_path.split('.')[-1]
    if extension_str.endswith('ipynb'):
        return
    err_msg = (
        'The extension is invalid. Please Specify a path of `.ipynb` extension.'
        )
    raise IOError(err_msg)
