An electrode-misalignment detection device in a resistance welder that joins members together by holding them between opposing electrodes and applying pressure and electricity thereto includes an electrode misalignment detector that simultaneously approaches outer peripheries of both electrodes to detect misalignment thereof. The electrode misalignment detector includes a clamping member, a signal generator, a non-closed-state detector, and a misalignment determiner. The clamping member is movable toward the electrodes to clamp them from opposite radially outer sides and is settable in a predetermined closed state when the electrodes are located at appropriate positions. The signal generator outputs a movement state signal according to a movement state of the clamping member. The non-closed-state detector detects that the clamping member is in a state other than the predetermined closed state based on the movement state signal. The misalignment determiner determines electrode misalignment if the clamping member is not in the predetermined closed state.