Each header tank includes a tank body and a header plate. The tank body includes a top wall, shorter side walls, and longer side walls. Each shorter side wall has joining portions integrally formed via rounded connection portions and extending along the inner surfaces of the longer side walls. The header plate includes a closure wall, shorter side walls, and longer side walls. End portions of each longer side wall of the tank body are curved along the rounded connection portions to form first bent portions. End portions of each longer side wall of the header plate are curved along the first bent portions to form second bent portions. The shorter side walls of the header plate have deformation portions which enter recesses formed between the rounded connection portions of the tank body and tip end surfaces of the first bent portions.