A reciprocating engine 1 includes an annular top ring 5 serving as a first piston ring disposed adjacent to a top surface (head end surface) 4 of a piston 3 defining a combustion chamber 2; an annular second ring 6 serving as a second piston ring disposed with the top ring 5 interposed between the annular second ring 6 and the top surface 4; an annular gas chamber 7 defined by the top ring 5 and the second ring 6; and a plurality of communicating passages 8 for allowing the annular gas chamber 7 and the combustion chamber 2 to communicate with each other. The top ring 5 and the second ring 6 are respectively inclined with respect to an X direction in which the piston 3 reciprocates, so as to be located further away from each other on a thrust side 9 than on an anti-thrust side 10.