Disclosed is a dummy shaft which prevents washers from falling off. In addition, disclosed are an apparatus for assembling a differential unit using the dummy shaft, and a method for manufacturing the differential unit. Specifically disclosed is a dummy shaft for temporarily assembling a differential unit by being inserted into a pinion gear, and a washer in contact with an outer surface of the pinion gear, including a body portion, and a movable portion slidable in a direction toward or away from the body portion. The movable portion has a receiving groove for catching the washer.