To compactly record a stable period for a long time by storing a measurement value and a time value in a data area only when the measurement value is judged to be different from a reference value and rewriting the reference value by the measurement value and preparing a next reference value.

First data are sampled at time t1. When the initial value of the reference value differs from the measurement value a1, a number '0' is stored in the data number 1 of the data area as the measurement value a1 and the initial value of time. Then, the reference value is rewritten by the measurement value a1. At time t2, second data are sampled. When a difference exceeding a permitted value which is previously decided is recognized as the compared result of the measurement value b1 and the reference value, the measurement value b1 and a time difference t2-t1 are stored in a data number 2 and the reference value is rewritten by the measurement value b1. When the differences from the reference value on the measurement values c2-c4 are not more than the permitted value in fourth-sixth samplings, the time differences t4-t3 to t6-t5 are added to the time value of a data number 3 and the time value of the data number 3 is set to be t6-t2.