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.