To perform updating of maximum values and minimum values of measurement data with a simple procedure without incurring an increase in the computational load of an arithmetic processing element such as a microcomputer. When processing is started, a most recent maximum value stored in a nonvolatile storage element is written to a maximum value-use variable Xmax and a positive maximum value is written to a minimum value-use variable Xmin. Each time temperature data is acquired, a value of acquired data Xk and a most recent minimum value Xmin are compared and the smaller value is set as a new minimum value Xmin. Each time updating of this minimum value is repeated a predetermined number of times of processing Ns, the minimum value Xmin at that point in time and the maximum value Xmax are compared and the larger value is set as a new maximum value Xmax.