Spot welding system including a welding gun with an electrode pair actuated by a servo-motor, a robot carrying the welding gun or objective workpieces and operating to change an relative positions and orientations of the welding gun and an workpieces, and a gun motion controller controlling the servo-motor to actuate the electrode pair. The gun motion controller includes a motion commanding section outputting motion commands to the servo-motor in accordance with a speed set-value designating a closing speed of the electrode pair, and a set-value adjusting section automatically adjusting the speed set-value to an appropriate value. The set-value adjusting section includes a force-data obtaining section obtaining force data representing, in a time-series manner, the pressurizing force generated in the electrode pair when the servo-motor actuates the electrode pair to close, and an appropriate-value computing section determining the appropriate value of the speed set-value based on pre-convergence transient data in the force data.