Abstract: An oil cooler includes a heat exchanger core having a plurality of plates laid one upon another to define each of an oil passage through which oil flows and a coolant passage through which coolant flows, between the plates. An uppermost plate of the plates has a core coolant inlet and a core coolant outlet. The core coolant inlet and the core coolant outlet are communicated with the coolant passage. A flow passage control valve is disposed on an uppermost part of the heat exchanger core and has a first coolant introduction opening through which coolant at a relatively high temperature is supplied and a second coolant introduction opening through which coolant at a relatively low temperature is supplied. The first coolant introduction opening and the second coolant introduction opening are changed over to be communicated with the core coolant inlet.