PURPOSE: To make the work of spraying concrete onto the corner part of a support and the cylindrical inner surface of its middle part, by calculating the locus of the position of the arm of a robot corresponding to the locus of a cylindrical surface along which a nozzle is to be carried, and inducing said nozzle along the excavated surface of a tunnel.

CONSTITUTION: For performing the work of automatically spraying concrete or the like onto the cylindrical inner surface of a tunnel, the inclination of a main swinging shaft 4 is adjusted in a manner such that the axis of said main swinging shaft 4 for the arms A1, A2 of a robot is made parallel with the axis of the tunnel. Then, the positions of two horizontal points A, B on a standard plane rectangular to the axis of the tunnel, the position of one vertical point C between the points A and B, and an inward point D existent at a position in a predetermined distance inside one of the aforementioned three points in parallel with the axis of the tunnel are all instructed to a nozzle 19. The positions of the arms A1, A2 against a spraying cylindrical surface J are calculated on the basis of said data, to incline the nozzle 19 toward the corner part of a support with a predetermined angle and to perform spraying while carrying said nozzle 19 along a circular locus. Then, the nozzle 19 is carried along the cylindrical surface J to perform said spraying onto the excavated surface K of the tunnel.