To solve the conventional problem that a charge pump is required to be mounted separately in order to replenish an insufficient working fluid due to unavoidable leakage from the closed circuit although a variable displacement type hydraulic pump and a hydraulic motor are in fluid-connection with each other by the closed circuit constituted of a pair of main oil paths in a hydraulic actuator such as a hydraulic continuously variable transmission which causes an unavoidable increase in component cost and the deterioration of maintenability resulting from an increase in the number of part items.

A hydraulic pump 2 which is brought into fluid connection with a hydraulic motor 3 by a closed circuit is provided with a charge structure 28 for supplying a part of pressure oil discharged from the hydraulic pump 2 to working fluid replenishing oil paths 43, 46 in the closed circuit.