A profile of the casing 20 is formed into a circular pipe shape, and the two gas coolers 10a, 10b are integrated into one body so that the longitudinal directions of the respective gas coolers can be substantially parallel with each other. Due to the above structure, it is possible to make the coolant flow smoothly in the casing 20 and stagnation of the coolant seldom occurs. Accordingly, as boiling of the coolant can be suppressed, it is possible to prevent the heat transfer coefficient from being remarkably deteriorated. Further, it is possible to suppress the generation of cracks, in the tubes 11, which are caused by heat stress.