A received FM wave signal is subjected to quadrature detection to generate baseband I and Q signals. The baseband I and Q signals are converted into a sequence of samples of phase information. An FM-detection-result signal is generated in response to a variation between two successive samples of the phase information. A power of the baseband I and Q signals is computed. A decision is made as to whether the baseband I and Q signals are valid or invalid by referring to the computed power and at least one threshold value. Signal interpolation is implemented with respect to the FM-detection-result signal in cases where it is decided that the baseband I and Q signals are invalid.