PURPOSE: To enable detection of several types of alcohol content in a mixed liquid, by transmitting and receiving a microwave within a hollow resonator to detect a transmission power and a resonance frequency at a resonance point of the hollow resonator.

CONSTITUTION: An oscillation output of a microwave oscillator 1 has a modulation signal from a low frequency oscillator 7 overlapping a control voltage thereof and FM modulated slightly. A microwave is transmitted 22 and received 23 within the hollow resonator 2 to be supplied to a detector 3. A DC component indicating the quantity of transmission power is extracted with an LPF 4 from the detector 3 while an AC component is extracted with an HPF 5 as FM demodulation wave. A phase difference is detected 6 between the FM demodulation wave and an output signal of the low frequency oscillator 7 as reference to be compared 8 with a specified reference voltage, integrated 9 and then, overlapped by an FM modulation signal from the oscillator 7 with an adder 10 to be supplied to the oscillator 1. Then, each content of several types of alcohol can be measured in a mixed fuel from two outputs of the LPF 4 and the integrator 9.