This paper is concerned with the noise emitted by the surface of reciprocating internal combustion engines. At first, the relevance of characteristic values of cylinder pressure process to emitted noise was reviewed. Next, the transfer coefficient between the cylinder pressure spectrum and sound pressure spectrum of noise was estimated, and an assessment of four kinds of four-stroke engine as noise generating objects was made. Finally, by the aid of these results, an attempt was made to separate the combustion noise and the mechanical noise from the engine noise.