An oil cooler includes a plurality of tubes having both ends each fixed to a pair of header tanks in the length direction to form channels. The oil cooler further includes heat radiation fins, a first flange having an inlet pipe and an outlet pipe each coupled thereto in the length direction, a second flange coupled to any one of the header tanks in the length direction, and a bypass valve installed between the first flange and the second flange. The fixing bolts bolt the first flange, the bypass valve, and the second flange together while penetrating through the first flange, the bypass valve, and the second flange in the length direction.