PURPOSE: To provide a cooling device of transmission oil by which transmission oil is cooled so as to restrain oil temperature up in a small space and at low cost.

CONSTITUTION: This cooling device of transmission oil is constituted so that the heat receiving part 7a on one end, of a heat pipe 7 is extended in a tank of oil 6 positioned in the case of a transmission 1, the radiating part 7b on the other end is positioned outside the case of the transmission 1, and the radiating part 7b is cooled by running wind of a vehicle or forcedly cooled with a blower mechanism constituted by utilizing a parking brake drum 4.