Continuous measurement of respiratory impedance with very high precision is enabled by executing noise removal. An air vibration pressure by an oscillation wave obtained by frequency culling such that the oscillation wave has only frequency components left by the culling from a plurality of different frequencies, is applied by a loudspeaker 21 to the inside of an oral cavity. The pressure in the oral cavity is detected and the flow of breathing is detected. These signals obtained are Fourier-transformed by a Fourier transforming means 32 and, thereby, a spectrum is obtained. A breathing high frequency component that contributes as a noise is obtained by the extracting means 33 using a spectrum that corresponds to the frequency components culled from the result of the Fourier transformation. The breathing high frequency component is subtracted from the spectrum that corresponds to the frequency components left by the culling and, thereby, the oscillation wave component is extracted. A computing means 34 executes computation of dividing a pressure component by a flow component for each frequency for the result of the extraction and, thereby, the respiratory impedance is obtained.
