PURPOSE: To make it possible to measure water depth with a reference water level as a reference, by providing a reference position mark at the location where a scale member, which receives the horizontal projection of a laser beam, corresponds to the water depth at the time of a reference water level, and based on the displacement between the light projecting point and the reference position.

CONSTITUTION: In a displacement detecting part 41 of a water depth measuring means, a memeory 12, which stores the height of a reference position mark A, is provided. A photoelectric transducer element of a photosensor 9, which receives the projection of a laser beam 81 at the time of a reference water level, is made to be the reference mark A. The height of attachment is stored in the memory 12 from a controller 10. The height of the reference position mark A is outputted to a subtractor 11 from the value in the memory 12. In the subtractor 11, the height of the reference position mark is subtracted from the height of the laser beam 81, and the result is outputted. As a result, the depth at the time of the reference water level and the measured amount of change are added in an adder 14. The result is displayed on a display device 15 as a depth with the reference water level as a reference. The depth from the reference water level can be accurately measured in this way.