This paper deals with the determination of the main design parameters on the efficient .betha. type stirling engine for marine use having the rhombic drive mechanism. This studies are performed as following. (1) The characteristics of $eta$ type stirling engine, (2) The kinds of driving mechanism, (3) The structure of rhombic drive mechanism, (4) The reasons of making choice of the rhombic drive mechanism in $eta$ type stirling engines, (5) Ultimately the purpose of this paper is to determine the main design parameters of $eta$ type stirling engines for marine use having the rhombic drive mechanism. Finally, We can adapt the result of this paper in designing of $eta$ type stirling engine driven by the rhombic drive mechanism.