An air compressor includes a cylinder being fitted with a piston body and an air storage container formed integrally with the cylinder The air storage container defines therein an air chamber having a diameter greater than or equal to the inner space of the cylinder A pressure relieving mechanism is directly mounted to the air storage container rather than at one outlet provided at the air storage container, wherein one portion of the pressure relieving mechanism is located in the air chamber of the air storage container while the other portion of the pressure relieving mechanism extends out of the air storage container to be in contact with a push button provided on an external enclosure, whereby a user may depress the push button to reduce the air pressure within the air storage container.