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