To provide an air supply device for a bicycle tube prevented from the looseness of a cap for pressing a mouthpiece and the formation of a clearance between the tapered face of a valve rubber and the tapered face of the inner surface of a hollow screw shaft even with the permanent set of the valve rubber due to long-term use.

This air supply device comprises the hollow screw shaft 3 erectly provided at the bicycle tube 1, with a fixing nut 2 thread-fastened thereto; a mouthpiece 6 fitted into the hollow screw shaft 3, with the valve rubber 5 fitted to the outer periphery of the air supply port 4; and the cap 7 externally fitted and thread-fastened to the upper part of the hollow screw shaft 3 while inserting and guiding the upper part of the mouthpiece 6. A coil spring 8 serving as an elastic member is fitted to the outside of the upper part of the mouthpiece 6, and the upper end of the coil spring 8 is engagedly fitted to the upper part inner surface of the cap 7, while the lower end is seated on the upper end of a whirl-stop seat 11 engaged with a pair of opposed vertical grooves 10 vertically provided on the peripheral surface of the mouthpiece 6 from the upper end of the hollow screw shaft 3.