The purpose of the present invention is to provide a process for producing an olefin polymer, which can produce an olefin polymer having a higher molecular weight by lowering hydrogen concentration efficiently in polymerizing an olefin in the presence of hydrogen by use of a gas phase reaction vessel. There is provided a process for producing an olefin polymer comprising the steps of: using a gas phase reaction vessel; polymerizing an olefin by use of a catalyst for olefin polymerization in the presence of hydrogen; and adding a hydrogenation catalyst to a bed portion in the gas phase reaction vessel.