PURPOSE: To shorten the time necessary for a spot welding as much as possible.

CONSTITUTION: A command for applying pressure is issued from a controller 1 to a welding gun 3 and the time T0 necessary for completing compression is counted and held in the non-volatile memory 17 of the controller 1. Whenever a positioning action for spot-welding is started, the waiting time T-T0 from the start of movement to the output of a pressing signal is found in accordance with a moving time T necessary for positioning and the time T0 necessary for pressurizing and the waiting time T-T0 is divided by a treatment cycle iTP to be converted into the frequency Nd of execution of a series of axial interpolation treatments. When the positioning action is started, the number of times C of execution of the series of axial interpolation treatments is begun to count and when a counted value C reaches the number of times Nd of execution, a command for pressing the gun is issued. Consequently, the time when the positioning of the robot body 2 is completed is made to synchronize with the time when the pressing of a spot welding gun 3 is completed, the wastefulness of waiting time after the completion of positioning is eliminated and the operation time is shortened.