The invention can reduce a number of parts, make an assembly and a part management easy and make a setting of an interval in an axial direction easy. A coupling apparatus is constructed by a lock ring which is rotationally engaged with a side gear corresponding to one of a differential case and a side gear capable of relatively rotating with each other by a coupling portion, is movable in an axial direction and has engagement teeth capable of engaging and interrupting with engagement teeth of the differential case on the basis of the axial movement, and an electromagnet which executes a movement in an axial direction of the lock ring on the basis of a line of magnetic force passing through the engagement teeth.