PURPOSE:To measure a standing wave ratio with high accuracy even when leakage is generated from a signal generator by providing a phase shifter changing the phase of a transmission signal by definite quantity between an object to be measured and a directional coupler. CONSTITUTION:A phase shifter 6 changing the phase of a transmission signal by definite quantity is provided between an object 1 to be measured and a directional coupler 2. A detector 4 detects both of the reflected wave msin thetareflected by the object 1 to be measured and coming from the output terminal 22 of the directional coupler 2 and the leakage wave Asin theta coming from the input terminal 21 of the directional coupler 22 of the output of a signal generator 3. At this time, the difference 2X between the max. value A+X and min. value A-X of the DC of the output of the detector 4 changed in its phase by definite quantity by the phase shifter 6 is taken and only a reflected wave X is calculated from said difference and the standing wave ratio of the object 1 to be measured is measured. Therefore, even when there is a leakage wave, measurement can be performed with high accuracy without generating a measuring error.
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