Echolocation signals of dolphins (clicks) consist of short duration and high frequency ultrasonic pulses. Click intervals of dolphins in captivity are less than two-way transiting time of a target range. Click intervals of free-ranging dolphins are thought to provide underwater sensory range of echolocation. A/D converter driven by a data acquisition program on Windows 95 © with a small signal processing circuit (Clicker 45) enabled real-time data acquisition of click intervals. Clicker 45 converted clicks to rectangular signals which duration was 0.5 ms. The amplitude of the rectangular signals were in proportion to sound pressure levels of clicks. When the amplitude of the rectangular signals were more than the threshold level (127 dB re 1µPa), the serial time and the amplitude were measured by the data acquisition system whose accuracy were 5 mV and 100 ms, respectively. Consequently, the click intervals and sound pressure level in click trains could be retrieved automatically. The rectangular signals from Clicker 45 could be recorded by an ordinary band-limited data recorder, so that the present system could be applied not only for a laboratory works but also for an open sea observation of dolphins' sonar.