To increase the reliability of a monitoring operation and a control operation by discriminating an abnormality.

A first high-frequency generation means 1 and a second high-frequency generation means 2 by which a first high-frequency signal and a second high-frequency signal at mutually different frequencies are generated on the basis of a reference clock signal and a control signal are provided. A first mixing means 3 which mixes the first and second high-frequency signals is provided. A second mixing means 4 by which a signal measured through an object, to be measured, by the first high-frequency signal is mixed with the second high-frequency signal is provided. A comparison means 7 which waveform-shapes a mixed output from the first mixing means so as to output a reference signal is provided. A hysteresis comparison means 11 which waveform-shapes a mixed output from the second mixing means so as to output a signal to be measured is provided. A phase detection means 9 which gives the reference clock signal and the control signal, which measures the phase difference between the reference signal and the signal to be measured and which outputs a detection finish signal and a phase output signal is provided. The hysteresis width of the means 9 is set at a prescribed level. When the signal level of a signal via the object to be measured is lowered to a prescribed level or less, the signal to be measured is not output.