PURPOSE: To enable the spraying work of concrete, etc. without forming any spraying specks by moving correctly a spray nozzle along the surface of a cylinder to be sprayed in parallel with the axial center of a drift even when a robot is deflected from the axial center of the drift.

CONSTITUTION: The inner surface of a circular-arcuate drift is sprayed with concrete, etc. with a nozzle 19 equipped to the leading ends of a revolute arms A1, A2, and 16 of a drift 31 for spraying concrete, etc. provided at an optional position in the drift. In this case, positions of the optional three points A, B, and C in a plane vertical to the axial center O1 of a main revolving shaft 4 of the robot 31 are instructed by detecting the distances (a), (b), and (c) between each point and the axial center O1 and angles α, β, and γ from a specified plane passing the axial center O1 with the axial center O1 as a center. The tilt angle of the axial center O1 with respect to a horizontal line in the direction O of the axial center of the drift is also detected. The nozzle 19 is moved in the circular- arcuate direction along an optional plane vertical to the axial center O of the drift by using the positional data of the three points and the data of the tilt angles.