A spot welding system includes a position detection part which detects the position of a movable arm, a first calculation part which calculates the combined spring constant of the movable arm and a fixed arm based on a position when the movable arm is driven by a predetermined drive force and the drive force, a second calculation part which calculates the spring constant of the movable arm based on a position when the movable arm is driven by a predetermined drive force and the drive force, a third calculation part which calculates the spring constant of the fixed arm based on the combined spring constant and the spring constant of the movable arm, and an elastic displacement calculation part which calculates an amount of elastic displacement of the fixed arm based on the spring constant of the fixed arm and a drive force when welding is performed.