DESCRIPTION VEHICLE AC GENERATOR Technical Field [0001] The present invention relates to a vehicle AC generator mounted in a vehicle such as an automobile and particularly to the improvement of the cooling structure therefor. Background Art [0002] In a conventional vehicle AC generator, the coil end portions of a stator coil are cooled with cooling air produced through the rotations of cooling fans fixed on a rotor (for example, refer to Patent Document 1) . That is to say, as illustrated in FIG. 23, a pair of cooling fans 21 and 22 fixed to the respective axis-direction end portions of a rotor 1 fixed on a rotor shaft 11 rotates integrally with the rotor 1, so that cooling air A and cooling air B flow into apair of cases 31 and 32 through air inlets 314 and 324, respectively, provided in the cases 31 and 32. [0003] The directions of the cooling air A and the cooling air B that have flown into the cases 31 and 32 are bent from the axis direction to the radial direction by the centrifugal force produced by the rotations of the cooling fans 21 and 22; then, the cooling air A and the cooling air B flow along axis-direction end faces 411 and 421 of coil end portions 41 and 42 of a stator coil 40 and are discharged through air outlets 315 and 325, respectively, provided in the cases 31 and 32. The axis-direction end face 411 of the coil end portion 41 is cooled through heat exchange thereof with the cooling air A; the axis-direction end face 421 of the coil end portion 42 is cooled through heat exchange thereof with the cooling air B. [0004] [Patent Document 1] National Publication of International Patent Application No. WO2005/093929A1 Disclosure of the Invention Problem(s) to be Solved by the Invention [0005] In the conventional vehicle AC generator configured in such a way as described above, the coil end portions 41 and 42 of the stator coil 40 are cooled through heat exchange thereof with the cooling air A and the cooling air B that are discharged in the radial direction from the cooling fans 21 and 22. However, because the coil end portions 41 and 42 of the stator coil 40 are arranged in such a way as to extend in the axis direction from the respective axis-direction end faces of a stator core 5 and to be perpendicular to the cooling Air A and the cooling air B that flow in the radial direction of the stator core 5, the portions where the cooling air A and the cooling air B exchange heat with the coil end portions 41 and 42, respectively, are limited to the axis-direction end faces 411 and 421 of the coil end portions 41 and 42. Accordingly, there has been a problem that, in the conventional vehicle AC generator, the cooling of the coil end portions 41 and 42 is insufficient. [0006] The present invention has been implemented in order to solve the foregoing problem in a conventional vehicle AC generator; the objective thereof is to provide a vehicle AC generator in which the coolability of the coil end portions of a stator coil is raised, and the amount of cooling air is increased so that the overall coolability of the AC generator is improved. Means for Solving the Problem(s) [0007] A vehicle AC generator according to the present invention includes a stator core having a plurality of slots on the inner circumferential portion thereof; a pair of cases, one of which is disposed at one axis-direction end portion of the stator core and the other one of which is disposed at the other axis-direction end portion and that have respective contact faces that make contact with the corresponding axis-direction end faces of the stator core, at least one of the cases in the pair having an air inlet for taking in cooling air that enters the case and an air outlet for discharging the cooling air to the outside of the case; a plurality of through-bolts that fix the pair of cases integrally with the stator core and extend in the axis direction of the stator core on the outer circumferential surface of the stator core; a stator coil that is mounted in the plurality of slots and has coil end portions, one of which protrudes in the axis direction from the one axis-direction end portion of the stator core and the other one of which protrudes in the axis direction from the other axis-direction end portion; a rotor shaft pivotably supported by the pair of cases; a rotor that is fixed on the rotor shaft and disposed in an inner space of the stator core; and one or more cooling fans fixed on at least one axis-direction end portion of the rotor. In the vehicle AC generator, the contact face of at least one of the pair of cases is provided with a cooling air path that connects the outer circumferential surface of the coil end portion with the outer circumferential surface of the stator core and has an opening in the axis direction of the stator core at the outer circumferential surface of the stator core. [0008] Moreover, a vehicle AC generator according to the present invention includes a stator core having a plurality of slots on the inner circumferential portion thereof; a pair of cases, one of which is disposed at one axis-direction end portion of the stator core and the other one of which is disposed at the other axis-direction end portion and that have respective contact faces that make contact with the corresponding axis-direction end faces of the stator core, at least one of the cases in the pair having an air inlet for taking in cooling air that enters the case and an air outlet for discharging the cooling air to the outside of the case; a plurality