An oil cooler includes flat tubes layered together with a clearance, wherein cooling water flows through the clearance. Each flat tube includes a first plate, a second plate, and a fin plate held between the first plate and the second plate. The first plate is recessed to form a fin plate accommodation portion accommodating the fin plate, wherein a thin portion is formed outside of the fin plate accommodation portion in a longitudinal direction of the flat tube. An oil port is provided at the thin portion. Each flat tube includes a guide wall at a lateral periphery thereof, wherein the guide wall faces the oil port in a width direction, and projects in a layering direction. The guide wall, the thin portion, and a lateral wall of the oil port form a nozzle portion to guide cooling water in the longitudinal direction.