An air compressor including an air storage container, a main frame, a motor, and a cylinder fitted with a piston body, the motor and the cylinder being provided on the main frame, the motor capable of driving the piston to conduct reciprocating motion in the cylinder to produce compressed air, which is transferred to the air storage container, wherein the air storage container is made separately from the cylinder, has a closed end face, a surrounding wall extending from the closed end face to define an inner space that terminates at an open end face, the inner space serves to receive the compressed air produced in the cylinder; a tubular connection portion is formed on the first wall of the cylinder, the tubular connection portion defines therein an exit hole communicating with an inner space of the cylinder, the space of the exit hole serves as an auxiliary air chamber for storing compressed air, wherein the cylinder is provided with a surrounding wall and is connected with a coupling flange having two opposite sides, each of which is provided with an holder; the air storage container being provided with a plurality of outlets (63, 64) and provided on its surrounding wall with a coupling flange having two opposite sides, each of which is provided with an L-shaped hook defining a recess; whereby the air storage container is capable of being fitted with the cylinder and rotated to have the opposite sides of the coupling flange of the air storage container slide onto the holder of the cylinder, so that the air storage container is detachably mounted to the cylinder, characterized in that the coupling flange of the cylinder is provided on the end of the cylinder which is interconnected with the tubular connection portion and is opposite to the open end face of the cylinder, wherein the coupling flange of the cylinder is formed extending from the surrounding wall of the cylinder, with the coupling flange of the cylinder having two opposite sides, each of which is provided with an L-shaped holder defining a recess; the coupling flange of the air storage container being provided on its surround wall around its open end face; whereby the air storage container is capable of being fitted over the cylinder and rotated relative to the cylinder to have the opposite sides of the coupling flange of the air storage container slide in the recesses of the L-shaped holder of the cylinder and have the opposite sides of the coupling flange of the cylinder slide in the recesses of the L-shaped hook of the air storage cylinder, so that the air storage container is detachably mounted to the end of the cylinder being opposite to the open end face of the cylinder and encloses the tubular connection portion, the tubular connection portion defines on its outer surface an annular groove to be fitted with a seal ring; a plurality of tabs is formed on the free end of the tubular connection portion, the tabs being arranged in a ring in regular gaps, each tab being provided with spaced ribs on its inner surface; a plug having three coaxially aligned portions including a first round portion, a middle round portion, and a second round portion is fitted into a central space surrounded by the tabs and limited by the spaced ribs of the tabs, so that the plug may not move sideways, the diameter of the second round portion of the plug being more than the diameter of the exit hole but less than the diameter of the central space surrounded by the tabs, the compressed air produced in the inner space of the cylinder being able to flow into the inner space of the air storage container via the gaps between the tabs; one or more compression springs are provided in the air storage container to force the plug to seal the exit hole of the tubular connection portion when the air pressure in the cylinder is less than a predetermined pressure, one end of each compression spring being fitted around the first round portion while urged against the middle round portion, or fitted around the middle portion while urged against the second round portion; the length of the exit hole is greater than the permissible displacement of the plug being away from the tubular connection portion, so that the auxiliary air chamber can serve as an effective chamber for storing additional compressed air.