PURPOSE: A method and device for measuring a concentration by using multi-phase microwaves are provided to quantitatively measure a concentration of liquid samples such as sludge with the high reliability in the state noise is removed. CONSTITUTION: A method for measuring a concentration by using multi-phase microwaves is as follows. Multi-phase microwaves are irradiated and passed through the measurement object sample. Transmission waves respectively corresponding to basic transmission waves(S1) and a plurality of phase oscillation transmission waves(S2,S3) are detected so that amplitudes(Es1,Es2,Es3) of each received waves are measured. The averaged amplitude value of the received waves is obtained by taking an average of the measured amplitude values of the received waves. A value of amplitude difference between the amplitude values of the received waves received by irradiating the transmission waves to pure water and the averaged amplitude value is obtained. The transmission waves which wavelength and amplitude in an idle state are known is irradiated to reference samples which concentrations are known and which sort is the same with the measurement object sample. The same transmission waves are irradiated to the pure water so that the amplitude changes of each case are measured. The concentration of the measurement object sample is measured by multiplying a proportional factor known in advance by the value of the amplitude difference.