PURPOSE: To enhance the aux. brake ability using cheap configuration by relieving, when necessary, the discharge oil pressure of a hydraulic pump for a working machine from a solenoid relief valve of proportional type, loading the hydraulic pump, and allowing it to work as an oil pressure retarder.

CONSTITUTION: When a plurality of hydraulic pumps p2, p5 are driven by an engine E, the pressure oil is distributed to a plurality of outrigger devices e1-e4 through an outrigger expanding/contracting valve (f). If therein a retarder control lever is turned on, a controller 5 outputs a command current or a plurality of voltages 32-34 so as to put modulation valve 6 for PTO clutch into On states 61, 62, put a solenoid type proportional relief valve 7 into operating condition, and also put the outrigger expanding/contracting valve (f) into contracted status f1. Particularly the hydraulic pump p2 for working machine is loaded by the relief valve 7 so that the pump p2 works as an oil pressure retarder.