PURPOSE: To improve forecasting accuracy statistically with the passageof measuring time by appreciating a forecasted result based upon a forecasting function selected out of data for forecasting and correcting the forecasting function by a negative feedback loop in accordance with the forecasted result.

CONSTITUTION: A temperature measuring part 1 is a circuit having a temperature-sensitive element 4 such as a thermistor and measuring the temp. of a portion to be measured. A forecasting operation part 2 is a circuit for forecasting the temp. continuously at the time of heat balance, monitoring a signal 11 outputted from the temp. measuring part 1 continuously and determining a forecasting starting condition on the basis of the temp. and the time elapsed. After starting the forecasting, the temp. at the heat balance is always forecasted with a short time interval by using up-to-data information such as an elapsed time signal outputted from a time measuring circuit 6 included inside the operation part 2 and a negative feedback control signal outputted from a forecasting temp. monitoring circuit 10 in addition to a signal 11 from the measuring part 1. A forecasting signal 12 is continuously sent to a forecasted result display part 3 until heat balance to display the forecasting temperature visually.