PURPOSE: To reduce noises, and to allow light wt. and low cost, by decreasing the surface area of an exhaust manifold by integrally casting one portion or the whole of the exhaust manifold to a cylinder head or a cylinder block.

CONSTITUTION: In this engine, a cylinder block 2, a cylinder head 3 and an exhaust manifold 4 are integrally casted, and an exhaust port 8 is communicated with the exhaust manifold 4. The exhaust manifold 4 is mounted by using a flank of the cylinder block 2, thus substantially reducing the surface area at 30W50%. Consequently, a surface of a wall 4a of the exhaust manifold 4 does not function as a surface of generating noises becuase the surface 4a is not projected from a surface of the engine, and a surface of a wall 4b has high rigidity and is difficult to generate noises because the surface 4b is unified with the cylinder block 2, thus extremely reducing noises in the case when the engine explosion burns.