PURPOSE: To reduce torque burden on a motor of a rotation drive mechanism for an antenna, etc., to be mounted on an artificial satellite and decrease the weight of satellite by furnishing a dummy load on the rotor side of the motor so that the inertia moment of the rotor side of motor is balanced with the inertia moment of its antenna side.

CONSTITUTION: A dummy load 11 is mounted coaxially with the shafts of a gear 4 and motor 1. Therein the inertia moment of this dummy load 11 is set so that the reaction torque to the satellite body 7 nullifies from the torque generated by the motor 1 driving a pay-load 3 such as antenna, the wear torque of bearing 6, the inertia moment of rotor 2 of the motor, gears 4, 5, the pay-load 3, and dummy load 11, and the gear ratio of the gears 4, 5. Thereby it is only required to lessen the inertia moment of the pay load 3 in compliance with the gear ratio, and the satellite can be embodied light and small.