To provide a rotation cutter type excavation device used efficiently for excavating a trench even in very hard underground, particularly even in the case that the soil has a large transition area between a relatively soft soil and the very hard soil.

In the device for excavating trenches in the ground, a fixing means is provided with a plurality of deformable members mounted to the main surface of an upper frame, a means to spout into the expandable member a pressure fluid for letting an expandable member exert a force between the main frame and a wall part of the trench, and a means capable of letting the pressure fluid escape from the expandable member for preventing the force from transmitting between the main frame and trench.