PURPOSE: To measure the depth of the sea bottom with low transparency without error without using an ultrasonic sensor by gradually altering the direction of a laser light to be emitted from a laser oscillator to a horizontal direction and obtaining the angle of emission of the light at the time of peak intensity of the reflected light.

CONSTITUTION: A direction of a laser light 6 to be intermittently emitted from a beam collimator 2 to the sea bottom 5 at each unit time is gradually altered by a polygonal mirror 11 to be rotated by a motor 10. The mirror 11 is deflected by about 30 degrees at a right angle to a horizontal direction by a scanning motor 18, and the mirror 3 is simultaneously deflected similarly by a scanning motor 4 in synchronization with the motor 18. The reflected light 9 from the bottom 5a is detected by a detector 8, amplified by an amplifier 12, its peak value is held by a peak holding circuit 13 until an oscillator 1 outputs next laser light 6, added together with the emission angle of the laser light obtained by measuring the rotating angle of the mirror 11 by an encoder angle gauge 17 to a depth measuring mechanism 14, and the depth is obtained.