PURPOSE: To easily execute a setting work and also to cut with the high accuracy a penetrating part even in case of a large diameter pipe, by driving a cutting torch in accordance with the locus of a penetrating line along the surface of a main pipe, and cutting the penetrating part thereby.

CONSTITUTION: By a step S1, the necessary parameters of a diameter D of a main pipe 1, the diameter (d) of a branch pipe 2, etc., are inputted from an input part, and also these input values are checked by S2. Subsequently, by S3, the coordinate data of the penetrating part is derived at every prescribed pitch θ by calculating X, Y and Z, when a groove cutting is not executed, and (x), (y) and (z), when the groove cutting is executed. Next, the shaft center position of a branch pipe 2 on an X-Y plane to a robot is inputted to S4, based on which the coordinate data of the penetrating part is converted to the point data of the robot by S5. Thereafter, after passing through S6 and S7, the input of an input part is executed by S8, and by S9, a nozzle 13 is moved in accordance with the point data and cutting is started, and after the consecutive cutting, the cutting work is ended by S10.