PURPOSE: To improve cooling efficiency by providing a drain duct with an oil cooler, and a feed oil duct with a hydraulic motor, and operation-connecting a fan, for making air blow to the oil cooler, to the hydraulic motor.

CONSTITUTION: A drain duct 9 is laid around a position in proximity to a cooling hydraulic motor 15, and an oil cooler 30 is disposed opposedly to a fan 16. Return oil is force-fed in the drain duct 9 from a cargo duct 3a, a charging duct 14 and a hydraulic circuit for travel, cooled in the oil cooler 30 receiving air blow from the fan 16, then recovered into an oil tank T, and delivered again into the ducts 3a, 5a, 5b, 14 by the rotation of pumps 3, 4, 5. This oil cooler 30 is disposed behind the seat of a vehicle, and the pipings of the drain duct 9 and charging duct 14 are laid out optionally, so that the motor 15, the fan 16 and the oil cooler 30 can be disposed in a large space, and this enables the installation of the large-sized oil cooler 30.