To control a discharge oil quantity from a primary pump according to a speed reduction ratio when a secondary pump on the output side is driven so as to provide a target speed reduction ratio by means of a simple inputting operation by inputting a speed reduction ratio to a valve control circuit and providing a control circuit by which a control valve and a directional control valve are controlled optimally on the basis of the input information from a monitoring sensor.

When a discharge oil quantity per a cycle is 1L, 1/2L of oil in the oil fed to a control valve 23 is returned to a primary pump 1, while 1/2L of oil is fed to a secondary pump 11 via a directional control valve 24. As the capacity of the primary pump 1 equals to that of the secondary pump 11, the secondary pump 11 is rotated by 1/2 of the rotation when 1/2L of oil is supplied from a driving source. A discharge quantity of the secondary pump 11 also becomes 1/2L, and the oil is returned to the primary pump 1 via the directional control valve 24. The returned oil of 1/2L from the controlling valve 23 to the primary pump 1 and the returned oil of 1/2L to the secondary pump 11 converge together, and consequently, an oil quantity of 1L is returned to the primary pump 1. This cycle is continued, and a speed can be reduced to be halved.