A heat exchanger allows establishment of a flat space between first and second plates. The heat exchanger is inserted in a closed circulating loop for coolant. The flat space is allowed to have a cross-section larger than that of a cylindrical duct of the closed circulating loop. The flat space serves as a flow passage. An increased cross-section enables a reduction in the flow speed of the coolant. The coolant is allowed to slowly flow through the flat space. The coolant thus contacts with the first and second plates for a longer time. The heat of the coolant is sufficiently transferred to the first and second plates. The efficiency of heat radiation is enhanced.