The reduction of friction and wear of cam/tappet interfaces in the valve gear mechanism is effective to improve the performance and reliability of internal combustion engines. Lightweight, antiwear materials such as ceramics seem applicable to the tappet, so it is necessary to study the tribological characteristics. In the present study, the frictional force between the cam and tappet is directly measured using a load sensor installed in a test rig, and the following is carried out. Firstly, the changes of friction with valve spring load, camshaft revolution speed and oil temperature are examined for current materials and CD-level oil of diesel engines, and their empirical relations with the coefficient of friction or frictional mean effective pressure are investigated. Secondly, the influence of oil contamination is examined for the oil used. Lastly, the friction characteristics of the ceramic tappet of silicon nitride, silicon carbide and sialon are obtained, and the influence of surface roughness and the running-in effect of rubbing are clarified.