An improved air compressor includes a cylinder that is fitted with a piston body and defines at its top wall a plurality of exit holes having different diameters and communicating between the cylinder and an air storage container. The exit holes are sealed by plugs and compression springs. The exit holes allow the compressed air produced in the cylinder to enter the air storage container more quickly. When the piston body approaches the top wall of the cylinder, one exit hole, with a smaller diameter, allows the compressed air to enter the air storage container more easily. The exit holes with different diameters allow the piston body to move in the cylinder more smoothly, and thus the efficiency of inflating an object can be increased.