To provide a cooling oil circuit having a wet type multi-disc brake embodied compactly at a low cost.

The cooling oil circuit having a wet type multi-disc brake includes a pump which circulates the cooling oil from the inlet of the brake to the outlet for cooling the brake, in which the oil suction port of the pump is put in communication with the oil outlet of the brake and the discharge port of the pump and the oil inlet of the brake are put in communication with a tank for draining the cooling oil. An alternative constitution is equipped with a pipeline through which an oil inlet provided on the minor diametric side of the brake and an oil outlet provided on the major diametric side of the brake are connected directly to the place between the above-mentioned oil inlet and outlet.