To provide a calibrator for a thermometer suitable for field calibration of the thermometer or operation test of a temperature relay carried out principally in a factory or a power station.

The calibrator 10 for thermometer comprises a measuring water tank 12 of heat insulated structure for inserting a standard thermometer 28 and a thermometer 30 to be measured, wherein the temperature of water in the measuring water tank 12 is regulated by injecting previously heated or cooled water. Previously heated or cooled water is preferably injected by means of a water supply pump 20 and water flowing over the measuring water tank 12 is circulated and reused. Cold water is injected after being diluted and the measuring water tank 12 is set in a temperature controlled water tank 18.