disclosed is to provide a process for producing an olefin polymer by continuous polymerization according to which the transition metal compound and the co - catalyst component promptly contact after they are fed to the polymerization reaction tank . the above process for producing an olefin polymer comprises continuously polymerizing an olefinic monomer in the presence of a polymerization catalyst obtained by contact treatment of a transition metal compound and a co - catalyst component in a polymerization reaction tank in which an agitation shaft having an agitating blade is suspended in the central part of the tank .