Recent years have seen much effort being expended to increase engine output while reducing engine volume, to achieve better fuel economy. In increasing engine output, the lubricating conditions of engine sliding areas come to be strict. It becomes important, therefore, to define the critical values of lubrication for ensuring engine reliability.In these aspects, a new method to measure piston temperature of the reciprocating engine was developed. This method employs a torsion coil spring as contact which sends out signals of piston temperatures. This contact has superior response and durability and being compact, makes it possible to take multi-channel measurement.In addition, it is demonstrated that this method for measuring sliding surface temperature permits evaluation of durability with con rod big end bearing and piston lateral face in a short time.