To eliminate the fluctuation of the number of revolution of respective ahydraulic motors due to oil leakage brought when a plurality of hydraulic motors are connected by transmission means to give an interlocking mition and connected in series in a hydraulic circuit.

The driving shafts 4, 5 of a plurality of motors 2, 3 are connected to a output shaft 7 by a transmission means 6. The output shaft 7 is connected to an earth auger or the like. The hydraulic motors 2, 3 are connected in sequence from a pressure oil source P by a series hydraulic circuit 9. Respective hydraulic motors 3 except the hydraulic motor 2 directly connected to the pressure oil source P are connected respectively to auxiliary circuits 10 diverged from the source P.