An oil cooler is held by holding plates. Top portions of side pieces of each holding plate are disposed through a side wall of a tank and are bent at the outer surface of the tank, so that circular interposed members and the oil cooler are retained by being sandwiched between middle supporter pieces of the holding plates and the side wall of the tank in the stacking direction of element units such that the oil cooler is movable in the longitudinal direction with respect to the side wall of the tank. By inserting connecting pipes of the oil cooler through openings of the tank so as to temporarily assemble the connecting pipes into openings of a tube plate, blocking flanges of the connecting pipes are retained while being in contact with the outer surface of the side wall of the tank.