PURPOSE: To make it possible to effectively operate with a simple structure by aligning a reflecting photodetector in the moving plane direction of a display member, and deciding the difference of surface reflection occurring in the case of protruding a display member to an optical path.

CONSTITUTION: A housing 9 is mounted at a cover head 3, rotated as required to dispose the opening 11 of a photoconductor 7 at the scale section of a desired switching temperature, and a significant optical or acoustic signal is generated at this position to notify the necessity of switching a heat source to a lower boiling stage to a user. A display member 2 is moved from a starting temperature to its predetermined switching temperature in the case of starting the fermenting step of fermentation substance existing in a fermenting tank. When the display part of the member 2 is arrived at the opening 11 of the photoconductor 7 at the photodetector 7, the difference generated at the surface reflection is decided. Then, a message signal is supplied from a sensor 5 to a signal generator 6 to generate the desired significant optical or acoustic signal.