An internal combustion engine performing its cooling by means of coolant and being capable of detecting a temperature of the coolant accurately is to be provided. In this engine, there is mounted a water outlet 3 provided with a temperature detecting means 9 for detecting a temperature of the coolant after cooling the engine, and with a rib 7 and an upslope surface 8 both of which are capable of guiding the coolant to a temperature detecting portion 9a of the temperature detecting means 9.