A process for producing an olefin polymer which Comprises homopolymerizing or copolymerizing an olefin in a gas phase by using a catalyst system comprising the following components A) and B):A) a solid catalyst component prepared by immobilizing a catalyst component comprising at least titanium, magnesium, halogen and an electron-donative compound on a porous polymer material, said solid catalyst component satisfying the following conditions (a), (b) and (c):(a) ethylene is preliminarily polymerized thereon in an amount of 50-200 g per gram of the component previously immobilized on the porous polymer material.(b) the preliminary polymerization of ethylene is carried out at most at a rate of 3 g/g-immobilized component-hour at least until the quantity of the preliminarily polymerized ethylene reaches 5 g/g-immobilized component, and(c) the preliminarily polymerized ethylene polymer has an intrinsic viscosity [.eta.] of 0.5 dl/g to 2.5 dl/g as measured in tetralin at 135.degree. C.; andB) an organoaluminum compound.Thus, an olefin polymer small in the extent of adhesion to polymerization reactor, small in the number of fish-eyes, low in the content of fine powder, high in bulk density and good in fluidity could be produced.