A miniature motor has a tubular case (1) closed at one end and a permanent magnet (2) fitted to the inner surface thereof. A case cap (6) closes the open end of the case, and a rotor (5) with an armature (3) and a commutator (4) is rotatably supported by bearings (9,10) provided on each end of the case (1). Brushes for making sliding contact with the commutator (4) are supported on the case cap (6). The atmosphere enclosed within the motor comprises a polymer having a molecular weight of 250 to 5000.