PURPOSE: To enable to remove a small amount of hydrogen gas as an impurity component especially by a simple process by bringing a gas containing CO as a main component into contact with an absorbent to dehydrate the gas, heating the dried gas and subsequently removing the hydrogen gas in the presence of a catalyst.

CONSTITUTION: A gas containing CO as a main component is brought into contact with an absorbent such as molecular sieve, active alumina or silica gel to remove water. The dried gas is heated at 50-350°C and further brought into contact with a catalyst preferably comprising ruthenium carried on active alumina, silica gel or any other carrier to remove a small amount (approximately 10-5,000ppm) of hydrogen gas contained in the gas.