To improve productivity while eliminating the need for adjustment work using an adjustment cap and keeping the operation accuracy of a relief valve high.

A relief valve includes: a cylindrical valve case 20 whose front end is attached to an air chamber of a compressor body; a valve body 22 arranged in the center hole 21 of the valve case; a spring means 23 for biasing the valve body in the closing direction; and a fixing cap 24 fixed to the rear end of the valve case. A pressure difference PD between the center hole and outside air is generated by making valve body operating pressure Pv lower than relief pressure P0 and forming an exhaust hole arranged in the fixing cap into a small hole like orifice whose diameter d is 2.0 mm or smaller. The relief pressure P0 is defined as the sum (PV+PD) of the valve body operating pressure Pv and the pressure difference PD.