To attain perfect sterilization with minimum calories and without defective sterilization by calculating heated calories from the measured value of temp. within a heated object measured every fixed time and judging the completion of sterilization when the total calories exceed a previously set calorie so as to stop sterilization work.

In a heating time judging circuit 81, when a timer 1 outputs the measuring command to a temp. measuring instrument 2 to measure temp. within the heated object G every fixed time, a temp. detection sensor S1 connected to this immediately measures temp. within the object G every fixed time and sends an analog signal to an analog/digital converter 3 to convert to a digital signal. This digital signal is sent to a calorie arithmetic unit 4 to calculate calories and these calories are integrated by a calorie integration device 5. Then this integrated value and the previously set value 6 are compared with each other and when the integrated value exceeds the set value, the completion of sterilization is judged. Then a controller 7 issues a command to a control valve 13 to stop the supply of a heat source.