To press a pressing element against earth and sand with a low drive power.

A reaction device is used as a device for transmitting a reaction force from a drilling machine for forming a hole in the ground to earth and sand around the drilling machine when the drilling machine is moved in the ground. The reaction device is composed of a casing 60 arranged in the rear of the drilling machine, a pair of pressing elements 62 arranged outside of the casing 60, peripherally extending and connected at its peripheral end to the casing 60 so as to be pivotable around an axis in parallel with the axis of a hole to be formed, and drive means 64 coupled to the other ends of the pressing elements 62, for displacing the other ends of the pressing elements 62 in directions in which they come to and away from each other.