An insulated bearing, comprising a pair of bearing rings (11, 13) arranged concentrically to each other and rolling elements arranged to be rotatable between the pair of bearing rings (11, 13), at least one of the bearing rings having an insulating layer formed thereon, wherein: the one of the bearing rings having the insulating layer formed thereon has an annular groove having an arc-shaped cross section formed on each of both axial end surfaces (25A, 25B) of the one of the bearing rings such as to be concaved toward an axially inner side, and at least a surface from a non-raceway surface side circumferential surface to the annular groove of the one of the bearing rings is covered by the insulating layer, each of the axial end surfaces (25A, 25B) of the bearing ring having the insulating layer formed thereon includes a non-raceway surface side end surface which is connected to the non-raceway surface side circumferential surface of the bearing ring, a raceway surface side end surface which is connected to an inner circumferential surface of the bearing ring, and the annular groove formed between the non-raceway surface side end surface and the raceway surface side end surface, and the annular groove and the non-raceway surface side end surface are connected to each other via an inclined surface, characterized in that an axial width between the non-raceway surface side end surfaces of the axial end surfaces (25A, 25B) is larger than an axial width between the raceway surface side end surfaces of the axial end surfaces (25A, 25B), the axial width between the raceway surface side end surfaces of the axial end surfaces (25A, 25B) is larger than an axial width between groove bottom portions of the annular grooves, and a portion of the insulating layer covering the inclined surface has a thinner thicknes (L2, L3, L4) than portions of the insulating layer covering the non-raceway surface side end surface and the annular groove. the one of the bearing rings having the insulating layer formed thereon has an annular groove having an arc-shaped cross section formed on each of both axial end surfaces (25A, 25B) of the one of the bearing rings such as to be concaved toward an axially inner side, and at least a surface from a non-raceway surface side circumferential surface to the annular groove of the one of the bearing rings is covered by the insulating layer, each of the axial end surfaces (25A, 25B) of the bearing ring having the insulating layer formed thereon includes a non-raceway surface side end surface which is connected to the non-raceway surface side circumferential surface of the bearing ring, a raceway surface side end surface which is connected to an inner circumferential surface of the bearing ring, and the annular groove formed between the non-raceway surface side end surface and the raceway surface side end surface, and the annular groove and the non-raceway surface side end surface are connected to each other via an inclined surface, characterized in that an axial width between the non-raceway surface side end surfaces of the axial end surfaces (25A, 25B) is larger than an axial width between the raceway surface side end surfaces of the axial end surfaces (25A, 25B), the axial width between the raceway surface side end surfaces of the axial end surfaces (25A, 25B) is larger than an axial width between groove bottom portions of the annular grooves, and a portion of the insulating layer covering the inclined surface has a thinner thicknes (L2, L3, L4) than portions of the insulating layer covering the non-raceway surface side end surface and the annular groove.