To provide a trench forming device capable of eliminating an inconvenience when a trench for burying a culvert pipe by cutting (excavating), easily forming the trench even when a lot of stones are present in a soil, and preventing the stones in the soil from being discharged to the surface of a ground on the outside of the trench.

This trench forming device comprises a work unit mounting part 15 installed on a runnable work unit through a rotating shaft in the horizontal direction, a trench forming part 16 having a raschel part 31 arranged continuously with the work unit mounting part on the underside and sharpened on the trench forming front side and recessed on the trench bottom side, and a drive means rotating the trench forming part about the rotating shaft to both wait positions where the trench forming part is positioned on the ground on the work unit side and on the ground on the anti-work unit side relative to a work position where the trench forming part is inserted into the ground.