To make it sure to monitor a harmonic having possibility of damage generation, by obtaining and accumulating the content value or the content ratio of a specified order harmonic component of a signal to be measured, at a specified judgment time and interval, and generating an alarm when the judgment level is exceeded.

A signal to be measured is inputted in an input circuit 1. An I/O 5 outputs a timing signal, and delivers input signal data to a CPU 9. A memory card 7 sequentially stores measured each order harmonic data. The CPU 9 executes various kinds of processes on the basis of the program of an ROM 10, the setting condition of a key input part 13, etc. That is, the CPU 9 obtains repeatedly the content value or the content ratio of specific order harmonic component of the signal to be measured, at constant time intervals in a specific judgment time, and accumulates the frequency. When the content value or the content ratio of the previously determined cumulative frequency exceeds a previously determined judgment level, an alarm is generated. Thereby generation of excessive harmonic level in a short time is surely detected without being affected by transient harmonic, and the alarm signal can be outputted.