A coolant circulation system for a vehicle-mounted internal combustion engine, the system comprising: a coolant circuit including a water jacket (36, 45) of an internal combustion engine; a motor-driven pump, which is provided in a middle of the coolant circuit and is configured to move coolant in the coolant circuit; a liquid temperature sensor, which is configured to detect a temperature of the coolant flowing in the coolant circuit; and a controller, which is configured to control the motor-driven pump, wherein the controller is configured to execute circulation stop control in which the motor-driven pump is not driven so that circulation of the coolant is kept stopped after the internal combustion engine starts up, the controller is configured to change a period during which the circulation stop control is executed in accordance with a temperature of the coolant detected by the liquid temperature sensor at the start of the circulation stop control, the controller is configured to execute variation determination control in which the motor-driven pump is driven during a predetermined period after the internal combustion engine starts up to move the coolant in the coolant circuit, thereby determining whether a variation in a temperature of the coolant in the internal combustion engine is equal to or less than a predetermined value based on the temperature of the coolant detected by the liquid temperature sensor, and the controller is configured to execute the circulation stop control on condition that it is determined in the variation determination control that the variation in the temperature of the coolant is equal to or less than the predetermined value. a coolant circuit including a water jacket (36, 45) of an internal combustion engine; a motor-driven pump, which is provided in a middle of the coolant circuit and is configured to move coolant in the coolant circuit; a liquid temperature sensor, which is configured to detect a temperature of the coolant flowing in the coolant circuit; and a controller, which is configured to control the motor-driven pump, wherein the controller is configured to execute circulation stop control in which the motor-driven pump is not driven so that circulation of the coolant is kept stopped after the internal combustion engine starts up, the controller is configured to change a period during which the circulation stop control is executed in accordance with a temperature of the coolant detected by the liquid temperature sensor at the start of the circulation stop control, the controller is configured to execute variation determination control in which the motor-driven pump is driven during a predetermined period after the internal combustion engine starts up to move the coolant in the coolant circuit, thereby determining whether a variation in a temperature of the coolant in the internal combustion engine is equal to or less than a predetermined value based on the temperature of the coolant detected by the liquid temperature sensor, and the controller is configured to execute the circulation stop control on condition that it is determined in the variation determination control that the variation in the temperature of the coolant is equal to or less than the predetermined value.