An internal combustion system includes a control device having an accumulated amount of time measuring unit that measures an accumulated amount of time by measuring an amount of time when the temperature of the coolant measured by a temperature sensor is equal to or higher than a defined temperature and accumulating the amount of time measured, an exchange determination unit that determines that the coolant needs to be exchanged when the measured accumulated amount of time reaches or exceeds an upper-limit accumulated amount of time, and an upper-limit amount of time setting unit that sets the upper-limit accumulated amount of time for determination by the determination unit in accordance with the type of metal forming the flow channel where the coolant flows.