PURPOSE: To enable overall downsizing without increasing work arms or trucks by providing a truck, rock drills, work arms and outriggers, and specifically providing a work arm with an outrigger which can face a tunnel skin.

CONSTITUTION: A truck 6 is lifted by a truck outriggers 8 in a proximity of a facing of a tunnel, and a rock drill 3 is moved by a work arm 4 to an optional position, corresponding to the position and the angle of boring. An outrigger 5 which is provided to the work arm 4 is faced to the facing of the tunnel. When an upper part or a side of the facing is bored, an outrigger stopper 46 is faced to and bound by timbering installed in the upper part or the side of the facing of the tunnel. When a bottom part of the facing of the tunnel is bored, the outrigger stopper 46 is faced to and bound by the bottom part of the facing of the tunnel. Then, the facing is bored with the rock drill 3, and the whole outer periphery of the facing is bored, and excavation is performed by non-blasting or low vibration blasting. Thus, boring work is stabilized and boring accuracy is improved.