PURPOSE: To prevent rise in the oil temperature, oil leaks or the like by changing a hydraulic pump to a variable displacement type so that the less the load is, the more the capacity will reduce.

CONSTITUTION: A hydraulic pump 8 is to be changed to a variable displacement type and a spring is to be provided in a hydraulic cylinder 20, which varies the capacity of the hydraulic pump 8, to give energy to the capacity reduction operating side of the hydraulic pump 8 and to induce the hydraulic fluid for the hydraulic pump 8 into a chamber opposite to the spring in the hydraulic cylinder 20. By doing so, the hydraulic pump 8 can be self-operated to the capacity reduction side less the load is, so a useless pressure oil circulation can be lessened and a rise in the oil temperature, oil leaks and such other matters can be prevented.