to the flow direction in the inflow passage 73 the first and second arc passages 75 and 76 are connected together to form the annular passage extending in a circle the inflow passage 73 extends along a tangent to the circle the outflow passage 74 extends in a diametral direction of the circle the inflow passage 73 is substantially perpendicular to the direction of the outflow passage 74 in this example the location of the water inlet 71 is away from but near the diametrically opposite position of the water outlet 72 in general a long passage has a greater flow resistance than a short passage therefore in this example the bend formed at the water inlet 71 between the inflow direction in the inflow passage 73 and the circumferential flow in the first arc passage 75 is made obtuse and nearly straight to reduce energy loss the bend formed at the water inlet 71 between the inflow direction in the inflow passage 73 and the circumferential flow in the second arc passage 76 is made relatively sharp to increase energy loss thus the water jacket 48 can make the flow resistances of the first arc passage 75 and the second arc passage 76 substantially equal to each other and thereby hold the flow rate in each of.