A position detection device for a workpiece to be welded including a spot welding gun having a pair of electrodes adapted to be disposed opposite each other across the workpiece; a robot for holding either one of the spot welding gun and the workpiece in a manner movable relate to each other; a servo motor for allowing the pair of electrodes to approach the workpiece and separate from the workpiece; a physical quantity detection section for detecting a physical quantity correlative to a torque of the servo motor when the servo motor allows one of the pair of electrodes to approach a surface of the workpiece so that the one of the pair of electrodes abuts against the surface of the workpiece; a position detection section for detecting positions of the pair of electrodes; a storage section for storing the physical quantity detected by the physical quantity detection section and a value detected by the position detection section; and a computation section for calculating a contact start time at which the one of the pair of electrodes comes into contact with the surface of the workpiece based on time-series data of the physical quantity stored in the storage section, and computing a position of the workpiece at the contact start time based on the value detected by the position detection section stored in the storage section.