PURPOSE: To operate an optimum equilibrium temp. by estimation, by detecting a state, such that a temp. gradient becomes gentle along with the rise in temp. and reaches a predetermined value or less at a place where the gradient becomes considerably small in the vicinity of the equilibrium temp., with a detecting circuit for stabilized temp. gradient.

CONSTITUTION: A temp., detection part 1 detects the temp. of a measuring area and sends out a temp. signal to supply the same to an operation part 2 which is constituted of a temp. threshold value monitor circuit 4, a temp. gadient operation circuit5, an estimation circuit 6 and a temp. gradient stabilizing and detecting circuit 7. The circuits 4, 5 receive the temp. signal from the detection part 1 and send out the same to the circuit 7 after predetermined operation. The circuit 6 performs predetermined operation upon the reception of the signals from the circuits 5, 7 and outputs the result of operation to a display part 3. By this method, optimum equilibrium temp. can be operated by estimation.