Because a low frequency of swallowing may lead to a worsening of oral aseptic conditions, it is considered necessary to maintain a normal frequency of swallowing to prevent aspiration pneumonia. We have measured the frequency of swallowing during the night using a portable sound recording system consisting of an IC recorder and a piezoelectric microphone. Using this system, the sounds of swallowing from the gular region were recorded from 2 subject and their sleepings. Recording sounds were clear and having little environmental noise The sound of swallowing was detected using an algorithm employing two digital Butterworth band pass filters. As a result, the frequency of swallowing during sleep was obtained. The developed system shows promise in the non-invasive measurement of the frequency of swallowing.