PURPOSE: To attain to conserve electric power and to miniaturize an electromotor to be used, by driving a power recovering fluid motor by sending a hydraulic fluid used as load compressed by a loading fluid pump to said motor to recover about 50% load energy.

CONSTITUTION: A loading fluid pump 4 for imparting load torque is connected to a rotary machine 2 to be tested and a flow amount and a compression pressure P2 are controlled to make it possible to test the same under an optional rotary number N2 and load torque T2. The loading fluid pump 4 sucks a hydraulic fluid 6 from a tank 5 through the suction pipe 7 from said loading fluid pump 4 to compress the same and the hydraulic fluid is sent to the suction side of a power recovering fluid pump 9 connected to a driving transmission electromotor 1 through the emitting pipe 8 from the loading fluid pump 4 to drive the same while returned to the tank 5 through the emitting pipe 10 of said power recovering fluid motor 9. In this case, the generated power of the power recovering fluid motor 9 is added to the rotary shaft of the transmission electricmotor 1 and the rotary machine 2 to be tested is drive by the sum of both outputs to make it possible to extremely conserve power required by the electric motor 1.