PURPOSE: To make it possible to miniaturize and thin a motor by arranging a structure in which the metal hub of a disk is directly chucked to the rotor yolk of a rotor by the magnetic attraction of the rotor magnets which drives the rotor to rotate.

CONSTITUTION: The magnetic flux of rotor magnets 4 leaks from the holes 2 punched on the chucking surface of a rotor yolk 1. The metal hub 11 of a floppy disk 12 is attracted by this leaking magnetic flux to the chucking surface 1a and mounted. At this juncture, a shaft 9 is coupled to the central aperture of the metal hub 11, but a chucking pin 3 is biased downward. Then, when the motor is rotated in this state to bring the chucking pin to the aperture for positioning, the pin is extruded upward by the resiliency of a flat spring 3a to be inserted in a chucking pin coupling hole 11a for coupling. When further rotated, the chucking pin 3 is positioned in the part on the outer diameter side where no cutting it is biased in the direction is then in a state that it is biased in the direction opposite to the coupling hole 11a.