A trench excavator capable of excavating a trench of a rectangular section suitable for formation of an underground diaphragm wall. The underground diaphragm walls are now applied as underground structures replacing caissons, rigid foundations for multistory or tower like buildings, underground tanks and the like. In the trench excavator according to the invention, a pair of drum cutters and a ring cutter interposed therebetween form one unit and a plurality of such units are arranged parallel to one another, whereby an unexcavated portion produced between the drum cutters can be cut out by the ring cutter, leaving no unexcavated portion.