To prevent an unbalance from generating in a clocking by a method wherein on one bank of a river, a light emitting-receiving means is vertically movably provided, and on the other bank, a reflecting means of a length corresponding with the vertical movement of the light emitting-receiving means, is arranged.

On one bank 3 of a river, a light emitting-receiving means 1 is arranged, and on the opposite bank 4, a reflecting means 2 comprising a vertically long mirror, is arranged. The light emitting-receiving means 1 is constituted of a light emitting means such as a photoelectric tube, and a light receiving means which receives the light reflected by the reflecting means 2, and is constituted in a manner to be vertically movable on a guide pole 6. The reflecting means 2 is formed to be larger than the moving distance in the vertical direction of the light emitting-receiving means 1 so that it does not need to be moved on a reflecting pole 5. When a float 16 vertically moves in a float guide 13 in response to a change of the water level of the river, the length of a hanging wire 10 varies in response to the movement. Then, the light emitting-receiving means 1 which is connected to the hanging wire 10 moves in the same direction as well, e.g. when the water level descends, the light emitting-receiving means 1 moves the descending direction. Therefore, the distance from the water level of the light emitting-receiving means 1 becomes uniform, and an always fair clocking can be performed.