Provided is an air compressor which helps to attain a proper discharge air temperature and which is superior in energy saving property. There are provided an air line connecting an air compressor, an oil separator, and an after cooler; an oil circulation line connecting the air compressor, the oil separator, and an oil cooler; a bearing oil supply line connecting one end of an intermediate branching portion disposed at an intermediate point of the oil circulation line between the oil cooler and the air compressor to a bearing oil supply portion of the air compressor; an intermediate portion oil supply line connecting the other end of the intermediate branching portion to an intermediate oil supply portion of the air compressor; a branching line supplying oil to the bearing oil supply portion and the intermediate oil supply portion; a blower sending air to the oil cooler and the after cooler; a bypass line connecting one end of a bypass branching portion disposed at an intermediate point of the oil circulation line between the oil separator and the oil cooler to the downstream side of the oil cooler of the bearing oil supply line; and a control valve controlling the inflow amount of the lubricating oil to the bypass line.