PURPOSE: To provide hydraulic circuits of a skid steer loader which can prevent pitching, jumping-out, a sudden stop and overturning.

CONSTITUTION: HST pumps 2 and 3 directly connected to an engine and HST motors 4 and 5 which are driven by these pumps and drive wheels, are provided, and hydraulic circuits 25 and 26 composed of orifices 21 and 22 and valves 23 and 24 connected in series to these orifices 21 and 22, are respectively connected in parallel to the HST motors 4 and 5. The valves 23 and 24 are operated so as to open by operation of a pedal 27. When the pedal 27 is operated, a bypass circuit of the HST motors 4 and 5 by the orifices 21 and 22 is formed, and since a change in rotating speed of the HST motors 4 and 5 to a quantity of operating fluid delivered from the HST pumps 2 and 3 becomes gentle, pitching, jumping-out, a sudden stop and overturning of a vehicle by a change in the operating fluid can be prevented.