Disclosed herein is a transmission oil cooler. The cooler includes heat exchange tubes stacked one on top of another. Each heat exchange tube comprises an upper plate and a lower plate coupled to each other. Recesses are formed in each of facing surfaces of the upper and lower plates. The recesses of the upper plate cross over the recesses of the lower plate. Ridges and valleys are alternately formed on the upper and lower plates. Each heat exchange tubes has, on opposite ends thereof, an inlet port and an outlet port. An upper flange is provided around each of the inlet port and the outlet port. A lower flange is provided around each of the inlet port and the outlet port. The upper flange and the lower flanges of the adjacent heat exchange tubes are coupled to each other by fitting.