To provide an automatic air supply mechanism for a pneumatic tire which can automatically supply air to a pneumatic tire by rotation of the pneumatic tire with respect to an axle when the air pressure in the pneumatic tire becomes a prescribed value or lower. There is provided an axle 101 and a wheel body rotatable with respect to the axle 101. An automatic air supply mechanism 1 is provided on the wheel body. The automatic air supply mechanism 1 has a constant pressure maintaining section 2 and a compression section 3. The constant pressure maintaining section 2 is configured to maintain the air pressure therein at a constant value and connected to the pneumatic tire mounted on the wheel body 110 in air flow communication. The compression section 3 compresses air when the wheel body 110 is rotated with respect to the axle 101 and feeds the compressed air into the constant pressure maintaining section 2.