A working fluid cooling control system has a controller 100 that receives inputs signals from a traveling motor speed pickup 101, a pressure sensor 102, a signal receiving line 103a of an option selecting switch 103 and a temperature sensor 104. The controller performs predetermined arithmetic processing and controls proportional solenoid valves 105 and 106. The pressures controlled by the solenoid valves are compared with positive control command pressures in shuttle valves 109 and 110. In this way, in the case of an operation pattern corresponding to a rise in temperature of the working fluid, minimum tilting angles of hydraulic pumps 11 and 12 are increased to increase an average flow rate of the working fluid passing through an oil cooler 40, thereby increasing an average heat discharge amount and reducing an equilibrium temperature of the working fluid.