A level ratio calculation circuit calculates average signal level data of signal level data corresponding to each sound collection beam signal, and calculates a level ratio between the average signal level data and each of the signal level data. Since a diffraction sound is substantially equal to all the signal level data, a diffraction sound component of the average signal level data also becomes substantially equal. On the other hand, a collection sound from a speaker is specific to the signal level data of the corresponding sound collection beam signal. Therefore, at the level ratio, the portion corresponding to the diffraction sound is flat and a data level becomes high locally in only the portion corresponding to the collection sound. By using this, the sound collection beam signal including the collection sound is detected.