To provide a method for manufacturing an olefin polymer, which is useful from the viewpoint of stably polymerizing or copolymerizing an olefin using a metallocene catalyst without forming a bulk polymer, can prevent the clogging of a polymer discharge pipeline with such a bulk polymer, and therefore can realize the continuous production of an olefin polymer or copolymer on a commercial scale.

The catalyst is conveyed and introduced into a polymerization reactor while an olefin or an olefin with a monomer is continuously charged into the reactor, wherein the catalyst is brought into contact with hydrogen to effect hydrotreatment before the catalyst serves to polymerize the olefin or copolymerize the olefin and a monomer copolymerizable therewith in the reactor.