An air compressor of compact design which has the capability of providing a relatively high pressure, relatively low volume output, and which is capable of operating over an extended range of speeds. A drive pulley is connected to a crankshaft structure and the crankshaft structure is connected to a reciprocating piston via a connecting rod. The piston reciprocates in a cylinder bore that has side inlets for admitting air and the piston compresses the air which is forced through a reed valve mechanism into an output chamber that communicates with an air pressure regulator for providing a constant desired output pressure.