When power output of automobile engine increases adverse effects resulting from increase of heat load on piston become serious. For a cooling countermeasure it is necessary to clarify the path along which heat is transfered and to be familiar with heat coming from the top surface of piston. In case of small pistons without a forced cooling system at the inside face of piston, most of the heat flows out of piston toward cylinder line through the sliding surface of piston ring and piston. Although it is well known that piston ring is very important as heat passage, details of a heat flow pattern still remain quite unknown. Some authors developed a few experimental studies, but we think it useful to realize a deep theoretical study of the problem. We think also very important to focalize thermal resistance of contact between ring surface and piston. The research results make evident that fluttering of piston way receives heat in a reciprocating way on rear and upper surface. A set of graphs represent thermal field change in twelve moments of operating cycle.