Air compressor including a box and an electrically operated compressor unit accommodated in the box, the compressor unit including a main frame, a cylinder fitted with a piston body, a motor, and a transmission mechanism, the motor and the transmission mechanism being mounted to the main frame, the motor driving the transmission mechanism to have the piston body conduct reciprocating motion in the cylinder to produce compressed air in an inner space of the cylinder, which is capable of being transferred to an air storage container provided with a plurality of outlets (62, 63, 64, 65), wherein the cylinder and the main frame are integrally formed of plastic, the cylinder defining an exit hole communicating with the inner space thereof characterised in that a metal seat defining therein a central hole is integrated in the cylinder, above the exit hole of the cylinder, and configured to endure high temperature within the cylinder during operation of the compressor unit and to thus to ensure air-tightness between a plug and the metal seat, the central hole of the metal seat communicating with the exit hole of the cylinder, the compressed air produced in the inner space of the cylinder being transferred to the air storage container via the exit hole of the cylinder and the central hole of the metal seat; the plug is provided in the air storage container and urged by a compression spring to seal the central hole of the metal seat when the air pressure in the cylinder is less than a predetermined pressure wherein the air storage container is a separate body from the cylinder and the cylinder is provided with a coupling flange having two opposite sides, each of which is provided with an L-shaped holder defining a recess; the cylinder is provided with a tubular connection portion on the top wall of the cylinder, wherein the tubular connection portion defines at its outer surface with an annular groove to be inserted with a seal ring and defines therein an exit hole communicating the inner space of the cylinder; the tubular connection portion has a top annular surface; the metal seat has a flared tubular projection and a flange formed at the bottom edge of the flared tubular projection, wherein the tubular projection has a top annular surface and defines a central hole communicating with the exit hole of the cylinder; the flange of the metal seat is embedded into the top annular surface of the tubular connection portion, so that the metal seat is integrated in the cylinder, above the exit hole of the cylinder; the inner space of the cylinder can communicate with the inner space of the air storage cylinder via the exit hole of the cylinder and the central hole of the metal seat; the plug, which is placed above the metal seat, can be urged by the compression spring to contact the top annular surface of the metal seat; the exit hole of the cylinder and the central hole of the metal seat are configured such that the sum of the length of the exit hole and the length of the central hole is greater than the permissible displacement of the plug being away from the metal seat, so that the exit hole of the cylinder together with the central hole of the metal seat is able to serve as an auxiliary chamber effectively for storing additional compressed air; the air storage container has a closed top and a surrounding wall extending from the closed top to define the inner space that terminates at an open bottom opposite to the closed top; the open bottom of the air storage container is provided with a coupling flange having two opposite sides, each of which is provided with an L-shaped hook, which is composed of a base section and an end section and defines a recess between the end section and the corresponding side of the coupling flange; the closed top of the air storage container is provided at its inner surface with a central column and an annular protrusion around the central column, thus defining an annular groove therebetween; one end of the compression spring is fitted around the central column and received in the annular groove, while the other end of the compression spring is urged against the plug; the air storage container is provided with a plurality of outlets (83, 84), which can be connected to various devices; the air storage container is capable of being fitted over the tubular connection portion of the cylinder and rotated about the cylinder to have the opposite sides of its coupling flange to slide in the recesses of the coupling flange of the cylinder and have the opposite sides of the coupling flange of the cylinder slide in the recesses of the coupling flange of the air storage container, wherein the L-shaped holders of the coupling flange of the cylinder and the base sections of the coupling flange of the air storage container are mutually blocked and thus the air storage container is detachably mounted to the cylinder.