PURPOSE: To provide a device capable of automatically detecting a contact state between two works in the case of a spot welding.

CONSTITUTION: A contact attaching plate 15 is provided at each circumference of electrode tips 3 so as to be elastically displaceable in the vertical-axial direction of a tip holder 2, and a pair of contact points 8, 9; 8a, 9a are attached to both contact attaching plates 15 to form a square with a vertical axis A as a center. A bridge circuit is structure for which a side to be measured on both sides is formed between points connecting one pair of contact points 8, 9 and the other pair of contact points 8a, 9a. The device is provided with an open state detector that detects the side to be measured is in an open state while a pair of contact points 8, 9; 8a, 9a are in contact with each other that belong to two sheets of work 5, 5a arranged between the electrode tips 3; an equilibrium state detector that detects the bridge circuit is in an equilibrium state; and a contact state discriminating means that produces welding OK signal when the open state detector fails to detect an open state and also when the equilibrium state detector detects an equilibrium state.