A reciprocal sound transmission system has been designed for long-term current measurements in the coastal sea with heavy ship traffic and fishing activities. The system was composed of two acoustic stations spaced a distance of 5.7 km on both sides of a channel in the Seto Inland Sea, Japan. Each station was equipped with a transmitter, hydrophone, and GPS receiver. Reciprocal sound transmission experiments between the two stations were successfully completed for 5 h, using a carrier of 11 kHz, modified with the M sequence of 10th order. The time coordinate at both stations was synchronized with the accuracy of 0.1 (mu)s by the 1-Hz and 1-kHz time signals of GPS. Range-averaged current velocities, estimated from the travel time data obtained reciprocally, were in good agreement with the results of the ADCP measurement obtained along the sound transmission line. A 10.6-km sound transmission experiment using the carrier of 11.0 kHz and the M sequence of 10th order was also done successfully in an adjacent channel of the Seto Inland Sea. The present sound transmission system can easily be extended to a coastal tomography system composed of an array of acoustic stations.