PURPOSE: To precisely detect operation frequency even if opening and closing valve operation is frequent by calculating the maximum value and minimum value of drift from the average value up to the last time, and making the average value zero when the frequency is without this range at the time of arithmetic processing of operation frequency of the opening and closing valve of a steam trap.

CONSTITUTION: When the frequency of opening and closing valve operation of a steam trap 12 is detected 10 as electric vibration for arithmetic processing 14, the maximum value and minimum value of drift are calculated from the average value of the sampling data up to the last time. When the sampling data of this time is the maximum value or more, or less than the minimum value, the average value is made zero, so that the operation frequency can be precisely detected even when the opening and closing valve operation is frequent. Hence, the drift width can be reduced, and the count of drift as the operation frequency can be prevent in the case of high pressure and thin thickness, and the judgment of a normally operating one as a defective can be prevented.