A water jacket includes: a cooling channel through which a coolant flows, the cooling channel being formed in a cylinder block in which a plurality of cylinders are formed such that the cooling channel is positioned outside the cylinders; and a cooling groove (slit) through which the coolant flows, the cooling groove being formed between adjacent ones of the cylinders. When a slit end portion is viewed in sectional view in a direction in which the cylinders are arranged, the slit end portion and a cylinder-block inner wall are continuous with each other via an inclined portion inclined so as to become thicker toward the cylinder-block inner wall.