A casing provided centrally with a sealed structure chamber. Small-sized separate four filter elements are arranged each on opposite sides of the chamber such that inflow air from a suction section into the casing is introduced via the filter elements into the chamber. Each filter element is detachably fitted over a filter-element mount having openings at its outer periphery. Upon filter-element replacement, the filter elements are replaced individually by opening a window. An integral cast casing having compressing sections and compressed air passages integrally fabricated therewith by casting is formed with receptacles for first and second intercoolers and an aftercooler. The intercoolers and aftercooler are received in the receptacles. Outlets of the intercoolers are connected with suction sides of second- and third-stage compressors through second- and third-stage suction pipes, respectively. A blowoff silencer is arranged between the integral cast casing and an oil tank.