A railcar bogie (201, 301, 401) comprising: a cross beam configured to support a carbody of a railcar, wherein said cross beam extends in a railcar width direction; a pair of front and rear axles between which the cross beam is disposed, the front and rear axles being respectively arranged in front of and behind the cross beam in a railcar longitudinal direction so as to extend in the railcar width direction; bearings respectively provided at both railcar width direction sides of each of the axles and configured to rotatably support the axles: axle boxes configured to respectively accommodate the bearings; coupling mechanisms configured to couple the axle boxes to the cross beam; plate springs extending in the railcar longitudinal direction so as to respectively support both railcar width direction end portions of the cross beam, each of the plate springs including two railcar longitudinal direction end portions each of which is respectively supported by one of the axle boxes; and an auxiliary supporting mechanism (210, 310, 410) configured to support at least one of the railcar width direction end portions of the cross beam in a case where said one end portion of the cross beam is displaced downward beyond a predetermined elastic deformation range of the plate spring, characterized in that the auxiliary supporting mechanism (210, 310, 410) includes stoppers (213, 214, 315, 316, 441) each configured to limit a rotation angle of the axle box around the axle therein within a predetermined angular range; in a case where the plate spring is within the elastic deformation range, there is a gap between each of the stoppers (213, 214, 315, 316, 441) and a portion (212, 314, 440) provided integrally with the axle box; and in a case where the railcar width direction end portion of the cross beam is displaced downward beyond the elastic deformation range of the plate spring, and this causes the axle box to rotate around the axle, each of the stoppers (213, 214, 315, 316, 441) contacts the portion (212, 314, 440) provided integrally with the axle box, to prevent the axle box from rotating, thereby supporting the railcar width direction end portion of the cross beam via the coupling mechanism. a cross beam configured to support a carbody of a railcar, wherein said cross beam extends in a railcar width direction; a pair of front and rear axles between which the cross beam is disposed, the front and rear axles being respectively arranged in front of and behind the cross beam in a railcar longitudinal direction so as to extend in the railcar width direction; bearings respectively provided at both railcar width direction sides of each of the axles and configured to rotatably support the axles: axle boxes configured to respectively accommodate the bearings; coupling mechanisms configured to couple the axle boxes to the cross beam; plate springs extending in the railcar longitudinal direction so as to respectively support both railcar width direction end portions of the cross beam, each of the plate springs including two railcar longitudinal direction end portions each of which is respectively supported by one of the axle boxes; and an auxiliary supporting mechanism (210, 310, 410) configured to support at least one of the railcar width direction end portions of the cross beam in a case where said one end portion of the cross beam is displaced downward beyond a predetermined elastic deformation range of the plate spring, axle boxes configured to respectively accommodate the bearings; coupling mechanisms configured to couple the axle boxes to the cross beam; plate springs extending in the railcar longitudinal direction so as to respectively support both railcar width direction end portions of the cross beam, each of the plate springs including two railcar longitudinal direction end portions each of which is respectively supported by one of the axle boxes; and an auxiliary supporting mechanism (210, 310, 410) configured to support at least one of the railcar width direction end portions of the cross beam in a case where said one end portion of the cross beam is displaced downward beyond a predetermined elastic deformation range of the plate spring, characterized in that the auxiliary supporting mechanism (210, 310, 410) includes stoppers (213, 214, 315, 316, 441) each configured to limit a rotation angle of the axle box around the axle therein within a predetermined angular range; in a case where the plate spring is within the elastic deformation range, there is a gap between each of the stoppers (213, 214, 315, 316, 441) and a portion (212, 314, 440) provided integrally with the axle box; and in a case where the railcar width direction end portion of the cross beam is displaced downward beyond the elastic deformation range of the plate spring, and this causes the axle box to rotate around the axle, each of the stoppers (213, 214, 315, 316, 441) contacts the portion (212, 314, 440) provided integrally with the axle box, to prevent the axle box from rotating, thereby supporting the railcar width direction end portion of the cross beam via the coupling mechanism.