A heat exchanger includes first, second, third, and fourth passages. Engine coolant flows through the first passages. Engine oil flows through the second passages. Transmission oil flows through the fourth passages after flows through the third passages. Each first passage and each third passage is disposed in the same layer. Each second passage and each fourth passage is disposed in the same layer. Each first and each third passage are disposed in a different layer from the layer of each second and each fourths flow passage. Each fourth passage is disposed upstream of a first flow direction of the engine coolant in each first passage, and each second passage is disposed downstream of the first flow direction. Each third passage is disposed upstream of a second flow direction of the engine oil in each second passage, and each first passage is disposed downstream of the second flow direction.