A production process is provided for an olefin polymerization catalyst component precursor, including the steps of (I) adding an organomagnesium compound to a solution containing a solvent, a Si—O bond-containing silicon compound, and a defined titanium compound, under agitation, and continuing the agitation until a magnesium concentration in a liquid phase of a reaction mixture decreases to 9 ppm by weight or lower, and (II) solid-liquid separating the reaction mixture. A production process is also provided for an olefin polymerization catalyst component using the above precursor. Further, producing process is provided for an olefin polymerization catalyst using the above catalyst component. Still further, a production process is provided for an olefin polymer using the above catalyst.