A driving force transmission apparatus comprising: an outer rotating member having an inner peripheral spline portion at an inner periphery of the outer rotating member, the inner peripheral spline portion having a plurality of inner peripheral spline protrusions (211, 212) extending in a direction along a rotation axis; an inner rotating member relatively rotatably supported coaxially with the outer rotating member, the inner rotating member having an outer peripheral spline portion at an outer periphery of the inner rotating member, and the outer peripheral spline portion having a plurality of outer peripheral spline protrusions extending in the direction along the rotation axis; a main clutch that includes an outer clutch plate engaged with the inner peripheral spline portion so as to be axially movable and an inner clutch plate engaged with the outer peripheral spline portion so as to be axially movable, and that couples the outer rotating member to the inner rotating member such that torque is transmittable; a cam mechanism that includes a first cam member and a second cam member, and that causes the second cam member to generate pressing force for pressing the main clutch by relatively rotating the first cam member with respect to the second cam member; and an electromagnetic clutch mechanism that includes an armature (50, 50A, 50B) engaged with the inner peripheral spline portion so as to be axially movable and an electromagnetic coil that generates electromagnetic force for axially moving the armature (50, 50A, 50B), and that applies rotation force to the cam mechanism for relatively rotating the first cam member and the second cam member with respect to each other at the time when the electromagnetic coil is energized, wherein the inner peripheral spline portion has a first region with which the outer clutch plate is engaged and a second region with which the armature (50, 50A, 50B) is engaged, the inner peripheral spline portion having a facing surface (212a, 212Aa) between the first region and the second region, the facing surface (212a, 212Aa) facing the armature (50, 50A, 50B), and movement of the armature (50, 50A, 50B) toward the first region is regulated by contact of the armature (50, 50A, 50B) with the facing surface (212a, 212Aa), characterized in that a sectional area of each of the plurality of inner peripheral spline protrusions (211, 212) in a cross section perpendicular to the rotation axis is smaller in the second region than in the first region, and a step face (212a, 212Aa) resulting from a difference in the sectional area is the facing surface (212a, 212Aa). an outer rotating member having an inner peripheral spline portion at an inner periphery of the outer rotating member, the inner peripheral spline portion having a plurality of inner peripheral spline protrusions (211, 212) extending in a direction along a rotation axis; an inner rotating member relatively rotatably supported coaxially with the outer rotating member, the inner rotating member having an outer peripheral spline portion at an outer periphery of the inner rotating member, and the outer peripheral spline portion having a plurality of outer peripheral spline protrusions extending in the direction along the rotation axis; a main clutch that includes an outer clutch plate engaged with the inner peripheral spline portion so as to be axially movable and an inner clutch plate engaged with the outer peripheral spline portion so as to be axially movable, and that couples the outer rotating member to the inner rotating member such that torque is transmittable; a cam mechanism that includes a first cam member and a second cam member, and that causes the second cam member to generate pressing force for pressing the main clutch by relatively rotating the first cam member with respect to the second cam member; and an electromagnetic clutch mechanism that includes an armature (50, 50A, 50B) engaged with the inner peripheral spline portion so as to be axially movable and an electromagnetic coil that generates electromagnetic force for axially moving the armature (50, 50A, 50B), and that applies rotation force to the cam mechanism for relatively rotating the first cam member and the second cam member with respect to each other at the time when the electromagnetic coil is energized, wherein the inner peripheral spline portion has a first region with which the outer clutch plate is engaged and a second region with which the armature (50, 50A, 50B) is engaged, the inner peripheral spline portion having a facing surface (212a, 212Aa) between the first region and the second region, the facing surface (212a, 212Aa) facing the armature (50, 50A, 50B), and movement of the armature (50, 50A, 50B) toward the first region is regulated by contact of the armature (50, 50A, 50B) with the facing surface (212a, 212Aa), characterized in that a sectional area of each of the plurality of inner peripheral spline protrusions (211, 212) in a cross section perpendicular to the rotation axis is smaller in the second region than in the first region, and a step face (212a, 212Aa) resulting from a difference in the sectional area is the facing surface (212a, 212Aa).