A supporting structure includes a metal axle tube, a holding member securely mounted in the axle tube, a supporting member securely mounted in the axle tube and including a supporting portion, and a fixing member securely mounted in the axle tube and including an opening. The holding member includes a hole and an inner flange is formed on an inner periphery defining the hole. A shaft is provided to a center of the rotor and includes an engaging groove. The shaft is extended through the hole of the holding member and the opening of the fixing member with an end face of a distal end of the shaft rotatably resting on the supporting portion of the support member and with the fixing member engaging with the engaging groove of the shaft, whereby the shaft and the inner flange of the holding member have a slight contact therebetween.