PURPOSE: To permit the excavation of a curved trench of the same width as that of a straight one by attaching an excavator having a cutter with a circular side shape to the front of a car body running in a freely vertically and horizontally turnable manner.

CONSTITUTION: When a wire 83 is wound up by a winch 40, a slide block 76b goes down along a guide 77 and an excavator 40 goes down. The excavator 40 is driven for excavation using a cutter 62, the downward operations of the excavator 40 is stopped after a given depth is reached, and a car body 2 is advanced for excavation of deep trench. When a curved trench is excavated, an oil-pressure cylinder 80b provided between the car body 2 and a bottom 79 is operated, the excavator 40 is turned through a desired angle around the shaft 78, and the rotation of the running wheel 10 is controlled for the excavation. Since the shape of the side of the cutter 62 is of circular arc, the excavation of a curved trench of the same width as that of straight one can be attained.