PURPOSE:To provide a transmission mechanism unit which can be easily operation-connected with the revolution equipment as driven device and develop the performance of a small-sized dc motor having a limitted capacity to the max. degree. CONSTITUTION:A planetary gear device P operation-connected with a small-sized dc motor is built in a cylindrical housing 3 on which the motor is fixed in demountable manner on one side. The planetary gear device P includes a revolution transmission member 4 which has a joint part 10 for the connection of the driving shaft of the rotary equipment as driven device, and the revolution of the motor 1 is deceleration-transmitted through the planetary gear device P to the driving shaft. Further, a plurality of inertia increasing mass bodies 11 are installed at proper positions on the revolution transmission member.