Abstract: Disclosed is a fuel cell system including a fuel cell stack, an enclosure surrounding the fuel cell stack, an air supply line connected to the fuel cell stack and configured to supply air to the fuel cell stack, an air compressor disposed in the air supply line and configured to compress and supply air, a cooler disposed in the air supply line, positioned at a downstream side of the air compressor, and configured to cool the air having passed through the air compressor, a bypass line having one end disposed between the air compressor and the cooler and connected to the air supply line, the other end connected to a separation space between the fuel cell stack and the enclosure, and a circulation line having one end connected to the separation space, the other end disposed at a downstream side of the cooler and connected to the air supply line.

Abstract: An air cooler includes: a cooler main body having a first zone and a second zone partitioned off from the first zone; cooling flow paths configured to cool the air and disposed in the first zone so that the air introduced into the cooler main body passes therethrough; bypass flow paths configured to allow the air to bypass the cooling flow paths and disposed in the second zone so that the air introduced into the cooler main body passes therethrough; and an opening/closing unit configured to selectively open or close the cooling flow paths, thereby obtaining an advantageous effect of simplifying a structure thereof and optimizing water balance of a fuel cell stack.