Abstract: A structure of a balance adjusting mechanism of the rotor casing of a miniature motor for driving a magnetic disc is provided. The miniature motor comprises: a hub carrying a driving magnet on its inner circumference and adapted to mount a magnetic disc on its outer circumference; a stator core having a coil wound thereon to face the driving magnet of the hub; a frame having a holder for fixing the stator core; a balance adjusting groove formed in the inner circumference of the hub and adjoining the driving magnet; and a balance correcting weight mounted in the balance adjusting groove.