A reciprocating fluid machine (2) includes pistons (32) reciprocated in respective cylinder bores (40). Each piston includes, as a lateral surface (32a) thereof, a cylindrical section (64) disposed in sliding contact with the corresponding cylinder bore with a coating layer (66) therebetween, ring groove sections (68) fitted with respective piston rings (34), and tapered sections (70) each formed between the cylindrical section and the corresponding ring groove section and inclined from the cylindrical section toward the ring groove section so as to be gradually set apart from the cylinder bore. The coating layer is also formed on each tapered section in such a manner as to be gradually set apart from the cylinder bore.