To enhance the stability of a depth-sounding data by holding vertical attitude of a directivity sensor constantly regardless of rocking of a surveying ship in an ultrasonic depth-sounding survey thereby preventing variation of the position on the sea bottom where an ultrasonic wave radiated from a directivity sensor strikes.

A directivity sensor 6 fixed with a float 4 to the upper part thereof is disposed, through a ginbal mechanism oscillating freely within a set angular range, in a sensor housing tank 3 filled with water having a bottom face made of an ultrasonic wave transmitting plate 9. Vertical radiation attitude is held by the buoyancy of the float within the set inclination angle range of the sensor housing tank 3 and the effect of acceleration on the directivity sensor 6 is suppressed since the directivity sensor is not secured.