A holding structure of a tire air pressure detecting device, comprising: a wheel in which a tire is mounted on a wheel rim; a tire valve which is fixed to the wheel rim; a tire air pressure detecting device which is held by a screw shaft protruding on an inner side of the tire in the tire valve; a fixing nut which is screwed to the screw shaft and configured to fix the tire valve to the wheel rim; a holding sleeve which is screwed to the screw shaft and configured to hold the tire air pressure detecting device on the screw shaft; and an elastic member which is sandwiched between the tire air pressure detecting device and a device supporting portion including at least one of the fixing nut, the holding sleeve and the wheel rim in an axial direction of the screw shaft, wherein: the tire air pressure detecting device is in a floating support state capable of being elastically displaced toward the fixing nut side in the axial direction of the screw shaft, the tire air pressure detecting device comprises a case side rotation restricting portion which engages with the elastic member in a case which accommodates equipment, and the elastic member comprises a rotation restricting portion which is configured to restrict rotation of the tire air pressure detecting device around the screw shaft by engaging with the case side rotation restricting portion, characterized in that the elastic member is integrally mounted on the fixing nut. a wheel in which a tire is mounted on a wheel rim; a tire valve which is fixed to the wheel rim; a tire air pressure detecting device which is held by a screw shaft protruding on an inner side of the tire in the tire valve; a fixing nut which is screwed to the screw shaft and configured to fix the tire valve to the wheel rim; a holding sleeve which is screwed to the screw shaft and configured to hold the tire air pressure detecting device on the screw shaft; and an elastic member which is sandwiched between the tire air pressure detecting device and a device supporting portion including at least one of the fixing nut, the holding sleeve and the wheel rim in an axial direction of the screw shaft, wherein: the tire air pressure detecting device is in a floating support state capable of being elastically displaced toward the fixing nut side in the axial direction of the screw shaft, the tire air pressure detecting device comprises a case side rotation restricting portion which engages with the elastic member in a case which accommodates equipment, and the elastic member comprises a rotation restricting portion which is configured to restrict rotation of the tire air pressure detecting device around the screw shaft by engaging with the case side rotation restricting portion, characterized in that the elastic member is integrally mounted on the fixing nut. the tire air pressure detecting device is in a floating support state capable of being elastically displaced toward the fixing nut side in the axial direction of the screw shaft, the tire air pressure detecting device comprises a case side rotation restricting portion which engages with the elastic member in a case which accommodates equipment, and the elastic member comprises a rotation restricting portion which is configured to restrict rotation of the tire air pressure detecting device around the screw shaft by engaging with the case side rotation restricting portion, characterized in that the elastic member is integrally mounted on the fixing nut.