An external heater operation determination system includes, in a vehicle: a first device to be heated by an attached external heater; a second device, separate from the first device, to be cooled by a refrigerant; a circulation device configured to circulate the refrigerant through a circuit; and a temperature detection device that is configured to detect a temperature of the refrigerant and disposed such that if the refrigerant is not circulating through the circuit, the temperature of the refrigerant detected by the temperature detection device in the circuit does not rise even if the external heater is operating, whereas if the refrigerant is circulating through the circuit, the detected temperature of the refrigerant changes. The external heater operation determination system includes a determination device able to determine that the external heater is not operating on a basis of a change in the temperature after the circulation of the refrigerant.