A pipe connecting member 7 has a fluid outlet passage 7B formed in communication with a fluid outlet pipe socket 721 provided thereon at a position on a lateral extension of the fluid outlet hole 9b, and a fluid inlet passage 7A formed with a fluid inlet pipe socket 722 provided thereon as displaced from a position on a lateral extension of the fluid inlet hole 7a. A corner portion 95 having a curved surface is formed between the fluid inlet passage 7A and a front header 4.