PURPOSE: To secure the safety of an offshore excavator by lessening the movement of the whole of the structure even in stormy waves by providing pillar legs made of a float which can be pulled up to a super-strcuture in addition to fixed pillar legs.

CONSTITUTION: In an offshore excavator in which a super-structure 4 is supported on a lower float 5 by three or more fixed legs 1, 2, and 3, two pillar legs 10 and 11 made of a float which can be pulled up on the super-structure 4 by a jack-up device 9 provided to the super-structure 4 are provided in addition to the fixed pillar legs 1, 2, and 3. Even when the semi-submerged type excavator having five pillar legs is submerged in water due to damage to any one of the pillar legs in the stormy waves of the open sea, for example, the safety of the excavator can be ensured by lowering the pillar legs 10 and 11 into water.