A first rotor (24; 124) and a second rotor (25; 125) are arranged in a coaxial and mutually rotatable relationship and are provided with a first driven gear (41; 141) and a second driven gear (42; 142), respectively. A drive shaft (31; 131) is also provided with a first drive gear (43; 143) and a second drive gear (44; 144) which are commonly connected to an output shaft of an electric motor (32; 132), and mesh with the first and second driven gears, respectively, at slightly different gear ratios. The first and second rotors are connected via a thread feed mechanism (36; 136) that converts a relative rotation between the first and second rotors into an axial linear movement between the first and second rotors that is used for changing a distance between a vehicle body part and a corresponding end of a suspension spring in a vehicle height adjusting system (9; 109). Owing to a differential rotation of a high gear ratio between the first and second rotors, a significant torque amplification is possible with a compact arrangement. The use of spur gears instead of a worm gear mechanism minimizes torque loss.