PURPOSE: To enable a simple and accurate forecasting by performing a computation for temperature forecast based on a data on an orthogonal polynomial coefficient ROM table.

CONSTITUTION: A quartz temperature sensor 1 is connected to an oscillation circuit 4 in an LSI3 and the oscillation signal thereof is divided into a low frequency with a frequency division circuit 5. The cycle of the signal of the temperature sensor thus divided is measured with a counting circuit 7 by a reference clock outputted from an oscillation circuit 6 using a crystal 2 for reference clocks. Measured values counted with the counting circuit 7 are inputted into a temperature forecast computation circuit 9, which accumulates measured values sequentially inputted and counts the final forecast saturation temperature using the measured values and an orthogonal polynomial coefficient ROM table 8. The forecast temperature value thus determined is shown on a display body 11 through a display body dividing circuit 10.