In order to recover the data obtained by various sensors deployed in the ocean, a study was made concerning the acoustic digital data communication system [Ochi et al., Proc. Meeting Marine Acoust. Soc. Jpn., pp. 85--86 (1996) (in Japanese)]. The data communication system was constructed by up to 99 of the same type of equipment in the area, and its specifications are: a half-duplex communication, FSK modulation, a 2500bps data rate, HDLC (high-level data link control procedure) protocol packet transmission, CRC (cyclic redundancy check) code, and frame retransmission for error detection/correction. The sea trial of this system was done at about a 4000-m depth area. Three sets of equipment were deployed in the ocean, and one of them hangs from the ship. Then, relay data transmission, among the three sets of equipment, was carried out. The detail of that sea trial will be shown at this presentation.