PURPOSE: To improve the extent of operation efficiency by installing each dry stroke detector in both hydraulic circuits of a main drifter stroking the rear end of a swing plate and a side drifter stroking the rear end of a side drill respectively, detecting a dry stroke and making it so as to give a warning to an operator.

CONSTITUTION: A field table 6 is installed, free of longitudinal motion, on a guide cell attached onto a rock surfacer via a mast free of vertical motion, and a main drifter 8 and two units of side drifters 9 are set up on the table 6. In addition, a line bit 14 is attached to the tip of a swing plate 10 and a drill bit 15 to the tip of a side drill 11, respectively, and a dry stroke detector 33 consisting of a hydraulic sensor is installed in each hydraulic circuit of these drifters 8, 9. With this detector, the occurrence of a dry stroke is caught as amplitude of working pressure, detected by the detector 33, lighting an indicator lamp 39, a warning is given to an operator. Consequently, the dry stroke is prevented from occurring, thus operation efficiency is improvable.