Abstract: An oil-free air compressor is characterized in an air inlet provided at a lateral open end of a crankcase of the air compressor. External air enters into the crankcase via the lateral air inlet and then enters into a cylinder above the crankcase via a check valve provided on a piston head in the cylinder. Air in the cylinder is compressed by the piston head and then discharged to an air storage tank via a single air outlet provided on a cylinder head of the cylinder.