An object of this invention is to provide a combination of a shim and a cam for use in a reciprocating mechanical system operated under a severe sliding condition, the combination not only assuring an excellent sliding characteristic by reducing friction work during sliding but also providing a high wear resistance and a high fatigue characteristic. In this combination of the shim and cam, the surface roughness of the sliding surface between the shim surface and cam is 0.08-0.4 &mgr;m in terms of ten-point average surface roughness Rz and the surface hardness of the shim sliding portion is higher than that of the cam and the cam has a plurality of cam mountains.