To facilitate the identification of a temperature change in issuing an alarm by setting an arbitrary reference temperature as an environmental temperature at a place which is about to be measured when a certain temperature change from the environmental temperature is found.

A data processor 7 is used to set an arbitrary temperature as a reference temperature. In cases where the absolute value of a difference between the reference temperature and a measured temperature is less than a certain value, data are processed with the reference temperature used as a threshold. In cases where the absolute value of the difference between the reference temperature and the measured temperature is equal to or more than the certain value, the data are processed with the measured temperature used as a threshold. Temperature distribution obtained through the data processing is displayed on a monitor 8. temperature.

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