there is provided a cooling water hole 10 at the bottom of each no 2 block partition wall 9 the open area of the cooling water hole 10 is made wider for downstream partition walls 9 the cooling water inlet 11 is located in no 1 block water jacket 8a while the cooling water outlet 12 is located in the no 2 block water jacket 8b in the first embodiment as indicated in fig1 and 3 the cooling water flows in a path as follows cooling water inlet 11 of cylinder block 6 the no 1 block water jacket 8a extending over full length of the cylinder block 6 head water jacket 2 of cylinder head 1 from intake port 13 side toward exhaust port 14 side each no 2 block water jacket 8b corresponding to each cylinder cooling water holes 10 cooling water outlet 12 of cylinder block 6 in the first embodiment thus arranged the cooling water flows following the above mentioned course and thereby cools the cylinder head 1 and the cylinder block 6 in this arrangement since an independent path of the cooling water is constituted for each cylinder and the open area of the cooling water hole 10 is made progressively wider toward downstream the volumes of the cooling water flowing through the water jackets 2 8 of.