Abstract: A main bearing case has a bearing holder for holding a bearing for supporting a crankshaft, and a crankcase mounting section at which it is joined to a crankcase. Spherical rib walls are formed on one face of the main bearing case opposite to the crankcase for connecting the bearing holder and the crankcase mounting section, on the upper side with respect to the axis of the crankshaft. Force radially exerted to the crankshaft is received by these rib walls, increasing the rigidity of the main bearing case, whereby deformation and play thereof are prevented.