To improve airtightness when a release hole is closed, and dispense with management of assembling accuracy of an unloading device in an air compressor in which a cylinder head is assembled onto a cylinder block through an intermediate plate, and a release hole formed on the intermediate plate is opened or closed by means of an unloading device.

An unload piston 24 is moved toward a release hole 20 at the unload time. A valve body sliding hole 27 communicated with the release hole 20 is formed on a valve body guide member 22 which is arranged on the way of an intake passage 8. An unloading valve 25 is inserted and fitted into the valve body sliding hole 27. The unload valve has one end as a valve head 31 which abuts against the valve body guide member 22 from the side of a cylinder block 2, and the other end abutting against the unload piston 24. A return spring 35 is elastically arranged between the valve body guide member 22 and the unload piston 25 for energizing the valve head 31 toward the valve body guide member 22.