In a heat exchanger, a refrigerant side header tank and a coolant side header tank which are connected with refrigerant tubes and coolant tubes include a plate header member, a communication intermediate plate member, a blocking intermediate plate member, and a tank header member. The communication intermediate plate member includes first and second fluid communication holes through which refrigerant and coolant flow, respectively. In this situation, with a simple configuration in that a part of the first and second fluid communication holes is blocked by the blocking intermediate plate member, a communication state between both tubes and internal spaces of the header tanks is regulated.