Japanese Laid-Open Publication Patent No. 40293/79 proposes a method for preparing a solid titanium catalyst comprising of magnesium, titanium, halogen and an electron donor, saying that a compound by a general formula, Ti(OR2)4 wherein R2 is a C1 ???C10 alkyl, aryl or cycloalkyl and a compound represented by a general formula, Mg(OR1)2 -n Xn wherein R1 is a C1 ???C10 alkyl; aryl or cycloalkyl, X