A resistance spot welding system includes a calculation unit that calculate and store a time variation of an instantaneous amount of heat generated, a division unit that divides a current pattern into a plurality of steps and stores a time variation of the instantaneous amount of heat generated and a cumulative amount of heat generated for each step as a target value, and an adaptive control unit that starts welding upon subsequent actual welding using, as a standard, a time variation curve of the instantaneous amount of heat generated that is stored as the target value, and adjusts welding current and voltage during welding, when a time variation amount of an instantaneous amount of heat generated deviates during any step from the time variation curve by a difference, so as to compensate for the difference during a remaining welding time in the step.