To sufficiently cool air exhausted in a discharge passage, while reducing manufacturing cost.

A cylinder body 12 of an air compressor is coated with a valve seat 20 composed of an iron-based or aluminum-based rolled plate material to close a cylinder chamber 13, and a suction valve 25 is arranged on a side end surface of the cylinder chamber 13 of the valve seat 20, and a discharge valve is arranged on an opposite side end surface. The valve seat 20 is coated with a cylinder head 30 composed of pressure metal mold casting and gravitational metal mold casting of aluminum, and the cylinder head 30 is provided with a suction passage 38 which communicates with the suction valve 25, the discharge passage 36 communicating with the discharge valve and a cooling water passage 49 for surrounding the suction passage 38 and the discharge passage 36. The cylinder head 30 is coated with a cover 70, constituted of the iron-based or aluminum-based rolled plate material for blocking up the cooling water passage 49. High-temperature air discharged from the discharge valve is cooled by cooling water of the cooling water passage. Since the valve seat and the cover are formed of the rolled plate material, manufacturing cost can be reduced.