PURPOSE: To enable actual horse power to be controlled in proportion to desired horse power by judging deviation of the actual horse power from the desired one according to symbols and converting the deviation to 0 when it has negative value, while energizing the deviation in the direction of increasing the inclination of a variable discharge controlling element of a variable capacity type pump.

CONSTITUTION: When each of discharge deviation Qf-Q, discharge pressure deviation Pf-P and horse power deviation Qf×Pf-HP has negative value, the output of a controller 10 has 0 value. Thus, a differential pressure valve 17 is quickly closed with the set pressure Ps of a spring 17e to locate a swash plate 1a of a variable capacity type pump 1 at the maximum inclination posi tion. As a result, the discharge Qf, discharge pressure Pf and horse power Qf×Pf will be quickly increase. Then, the discharge Qf, discharge pressure Pf and horse power Qf×Pf can be accurately controlled to the respective desired values only by sending the respective desired values to the input of the control ler 10 through setters 7W9.