Abstract: A resonant frequency characteristic in a resonant space is detected, based on a base amplitude frequency characteristic obtained by outputting a sound wave of a specified measurement signal from a speaker 13 disposed in a round space 40 and by receiving the sound wave in a microphone 14 disposed in the round space 40, a first amplitude frequency characteristic obtained by outputting, from the speaker 13, a sound wave of the measurement signal and a signal output from the microphone 14 and by receiving the sound wave in the microphone 14, and a second amplitude frequency characteristic obtained by outputting, from the speaker 13, a sound wave of the measurement signal and a phase inverted signal obtain by inverting a phase of the signal output from the microphone 14 and by receiving the sound wave in the microphone 14. The second delay time is different from the first delay time.