In a disk chucking mechanism for use in a flexible disk drive adopting a spindle motor (300) of a Hall sensor-less method, a driving pin (317A) freely received in a driving oval hole (46b) of a flexible disk (40) has a shape (shape of fan) where a catch factor portion is cut so that the driving pin (317A) is not in contact with an inner edge of the driving oval hole (46b) as much as possible when a rotor (310A) of the spindle motor (300) rotates in the opposite direction (counterclockwise).