PURPOSE: To improve the position determining accuracy of a target by oscillating circular-spot-shaped laser light or slit light obtained by converting this laser light with an oscillating mirror in the horizontal or vertical direction, and emitting the light into the water.

CONSTITUTION: A cylindrical lens 3 is provided on the light path of laser light 2. An underwater polarization light mirror 5 is fixed at an arbitrary angle. Then, the laser light 2 becomes a fan-shaped slit laser light 15, which is expanded in the horizontal direction with the lens 3. The laser light 15 is reflected with a vertical oscillating mirror (vertical deflection) 7 through the mirror 5 and emitted into the water. The mirror 7 is driven and oscillated with a gavanometer 8 in response to the angle signal from a motor controller 11. Therefore, the laser light 15 is made to sweep in the water in the vertical direction in response to the angle change of the mirror 7. When the light is cast on a target 15 by the sweep with the fan-shaped laser light 15 in the vertical direction, scattered light 14 is generated from the target 13. The scattered light is condensed on a photoelectric converter 10 through a condenser lens 9.