PURPOSE: To reduce a change in effective impedance of a detector even when a high frequency power to be measured increases or decreases by adding or interrupting a bias voltage of a detection diode according to a magnitude of the high frequency power to be measured.

CONSTITUTION: Two transistors T21 and T22 are used and a bias power source E is provided between the emitter and ground of one transistor T21 thereof. The emitter of the other transistor T22 is grounded directly. When the transistor T21 conducts, a bias voltage is supplied to a detection diode Di through the transistor. Gates of the transistors T21 and T22 are connected mutually through an inverter Iv. Any one transistor conducts according to polarity of a switching signal and the other transistor is interrupted.