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.