To improve cooling efficiency, and reduce size and weight by forming an oil tank to constitute a hydraulic device in a thin plate shape, and arranging a ventilating means to forcedly ventilate an end plate surface of the oil tank.

In a travel driving part 4 of a self-traveling type work device 2 having a hydrostatic transmission HST, an engine pulley 6 as a transmitting means is connected to an output shaft of an internal combustion engine 5, and an HST input pulley 8 as a transmitting means is also fixed. In order to cool a transmission 1, an axial flow type air blowing fan 10 is formed on this HST input pulley 8 as a ventilating means to forcedly flow and ventilate air in the vicinity of an oil tank 16. The oil tank 16 is formed in a thin plate shape, and its end plate surface 16a is molded so as to have the area enough to cover the contour on the air blowin downstrea side of the air blowing fan 10, and the end plate surface 16a is arranged oppositely to the air blowing fan 10 so as to receive blowing air by the air blowing fan 10 by the wide area.