In recent years, there has been a growing demand for acoustic communication equipment for collecting data from observation devices installed underwater and controlling remotely operated vehicles. With the objective of collecting data from these devices, we developed a packet transmission system based on frequency shift keying (FSK) modulation. This equipment is a half-duplex acoustic communication device with a frequency of 20 kHz±1.25 kHz, transmission rate of 1,250 bps and a maximum acoustic power of 20 W. Using this equipment, a trial was carried out in the sea at the depth of 3,500 m in October 1998. At a transmission distance of approximately 4,000 m, it successfully transmitted error-free 15-kbytes data.