PURPOSE: To transmit a secondary wave having a sharp directivity toward right beneath a ship's bottom when a hull is rolling.

CONSTITUTION: As a wave transmitting system, the following parts are provided. A transmitting-timing controller 1 outputs a trigger signal T when a rolling angle θ of a hull 8 is zero. Transmitters 2 and 3 output transmitting-pulse electric powers P (F1) and P (F2) having two close frequencies based on the trigger signal. A parametric wave transmitter 4 forms the primary waves having two close frequencies based on the transmitting-pulse electric powers, forms the secondary wave B (F1-F2), which is the difference between the primary waves, based on the parametric effect and transmits the secondary wave to the sea bottom. As a wave receiving system, the following parts are provided. A wave receiver 5 receives the secondary wave BR (F1-F2), which is reflected from the sea bottom and returned as an echo R, and outputs the echo. A receiver 6 receives the echo and the trigger signal T, obtains the water depth value of the sea bottom and the outputs the echo. A recorder 7 records the water depth value and the echo.