A part assembling apparatus is provided with a plurality of assembling portions and a ball screw shaft. Each of the assembling portion includes a nut runner for assembling a part to an assembling target. The ball screw shaft includes a thread portion. At least two of the plurality of assembling portions respectively include nut portions threadedly engaged with the thread portion. The nut portions are movable in an axial direction of the ball screw shaft. The thread portion includes a first thread part having a first pitch and a second thread part having a second pitch different from the first pitch.