A method and apparatus measuring temperature of a steel sheet. A reference plate including a temperature controller is disposed opposite to the steel sheet. The reference plate temperature of the reference plate is measured by a contact thermometer. A radiation pyrometer is trained on the steel sheet at an angle such that alternate reflection of radiation energy by the reference plate and the objective steel sheet occurs once or twice. The radiosity of the steel sheet is measured by the radiation pyrometer. A temperature obtained by converting the radiosity into a temperature of a blackbody that radiates energy equivalent to the radiosity is used as a radiosity temperature. The temperature controller executes a control operation to make the temperature of the reference plate coincide with the radiosity temperature. The radiosity temperature is used as the temperature of the steel sheet.