Abstract: A hot water injection apparatus for a water cooling engine having a cooling water main circulation line and a cooling water bypass circulation line comprises a thermal insulation vessel which is vertically installed. The vessel has an inlet opening and an outlet opening of which level is higher than that of the inlet opening. The inlet opening and the outlet opening are connected in series to the bypass line. When the engine is started, the high temperature water stored in the vessel flows into the engine to accelerate the warming-up operation. During the normal operation, the warmed water which is obtained by mixing the low temperature water in the main line with the high temperature water in the bypass line, flows into the engine to promote the vaporization of the fuel when the engine is stopped, the high temperature water in the bypass line is stored in the vessel and is held at a high temperature.