To provide an auger device for excavating a ditch by which the ditch can be efficiently excavated using an auger.

An auger screw 8 and a rotary casing 9 externally fitted to it are hangingly connected to a rotary drive part 7 of an auger body 6, which can be driven up and down along a leader 2 supported to a self-propelled vehicle 1, in a manner that they can be rotated and driven in opposite directions. The rotary casing 9 has a plurality of vertically long excavating windows 10 arranged at predetermined intervals in the peripheral direction and extending from a lower end part of the rotary casing to a predetermined height position. An excavating blade 11 is fitted to each of the windows 10 in an outwardly projecting manner and along a side edge part of a rotation directional rear side thereof. An excavated soil dispersion preventive plate 12 having an arc shape in cross section is integrally fitted to the auger body 6 to go half way around an outer periphery of the rotary casing 9 and on a rear side of a ditch excavation proceeding direction.