To provide a diaphragm-wall excavator carrying out excavation in various excavation width and the expanded excavation of the bottom section of a diaphragm wall.

The diaphragm-wall excavator prepares a diaphragm wall in a ground while excavating the ground. A cutter holder 9 is installed at the lower end of an excavator body 1 inserted into the ground horizontally movably in the horizontal direction, a pair of left-right cutter drums 4, 4 are mounted movably to the lower section of the cutter holder 9 in the horizontal direction mutually independently, and the front ends of these cutter drums 4, 4 can be projected from the external side face of the excavator body 1 to the outside.