A resistance spot welding device for squeezing parts to be welded, the parts to be welded being a plurality of overlapping metal sheets, by a pair of electrodes and, configured to apply an electrode force while passing a current through the parts to be welded to join the parts to be welded, the resistance spot welding device comprising: a storage configured to store an amount of time variation of an instantaneous amount of heat generated per unit volume as a time variation curve and to store a cumulative amount of heat generated per unit volume, both the amount of time variation and the cumulative amount of heat being calculated from an electrical property between the electrodes in the case of forming an appropriate nugget by passing a current by constant current control in test welding that precedes actual welding; an adaptive controller configured to start the actual welding using the time variation curve stored in the storage as a target and, in the case where the time variation of the instantaneous amount of heat generated per unit volume in the actual welding differs from the time variation curve, control the current or a voltage during current passage to compensate for the difference within a remaining welding time so that the cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume stored in the storage; and characterised by an electrode force controller configured to control the electrode force to the parts to be welded in cooperation with the adaptive controller to compensate for the difference within a remaining welding time so that the cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume stored in the storage, wherein the electrode force controller is configured to increase or decrease a subsequent electrode force in the actual welding, depending on a difference between the cumulative amount of heat generated per unit volume in the actual welding and the cumulative amount of heat generated per unit volume in the test welding when a predetermined time has elapsed from start of the current passage, and wherein the electrode force controller is configured to control a time from the start of the current passage to when the electrode force controller starts the control of the electrode force in the actual welding to fall within a range of 0.1 cycles to 10 cycles. a storage configured to store an amount of time variation of an instantaneous amount of heat generated per unit volume as a time variation curve and to store a cumulative amount of heat generated per unit volume, both the amount of time variation and the cumulative amount of heat being calculated from an electrical property between the electrodes in the case of forming an appropriate nugget by passing a current by constant current control in test welding that precedes actual welding; an adaptive controller configured to start the actual welding using the time variation curve stored in the storage as a target and, in the case where the time variation of the instantaneous amount of heat generated per unit volume in the actual welding differs from the time variation curve, control the current or a voltage during current passage to compensate for the difference within a remaining welding time so that the cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume stored in the storage; and characterised by an electrode force controller configured to control the electrode force to the parts to be welded in cooperation with the adaptive controller to compensate for the difference within a remaining welding time so that the cumulative amount of heat generated per unit volume in the actual welding matches the cumulative amount of heat generated per unit volume stored in the storage, wherein the electrode force controller is configured to increase or decrease a subsequent electrode force in the actual welding, depending on a difference between the cumulative amount of heat generated per unit volume in the actual welding and the cumulative amount of heat generated per unit volume in the test welding when a predetermined time has elapsed from start of the current passage, and wherein the electrode force controller is configured to control a time from the start of the current passage to when the electrode force controller starts the control of the electrode force in the actual welding to fall within a range of 0.1 cycles to 10 cycles.