PURPOSE: To make it possible t perform accurate measurement, in the underwater excavator, by computing the position of a blade body, from the position of a blade body supporting wire on the ground level and the depth of the blade part.

CONSTITUTION: The blade body part 4 is fixed to the lower end of a kelly bar 3 which is slidden in a guide 2 that is supported by a heavy machine 1. Under the state a horizontal direction controlling tool (k) (immobile point) of the bar is eccentrically moved to a point which is separated by a distance (a) from a reference line (c), the distance between the bar and the reference line at the ground level when excavation is performed to a depth l is made Δy. Then a distance (y) between the blade body part and the reference line can be computed by using the analogous relationship of a triangle based on (a), Δy, l, and (k).