such a catalyst system has no particular limitation and it is possible to use various known catalyst systems affording a highly stereoregular polypropylene such a solid catalyst component a can be prepared for example according to the processes disclosed in the following publications japanese patent application laid open nos sho 50 108385 sho 50 126590 sho 51 20297 sho 51 28189 sho 51 64586 sho 51 92885 sho 51 136625 sho 52 87489 sho 52 100596 sho 52 147688 sho 52 104593 sho 53 2580 sho 53 40093 sho 53 40094 sho 55 135102 sho 55 135103 sho 55 152710 sho 56 811 sho 56 11908 sho 56 18606 sho 58 83006 sho 58 138705 sho 58 138706 sho 58 138707 sho 58 138708 sho 58 138709 sho 58 138710 sho 58 138715 sho 60 23404 sho 61 21109 sho 61 37802 sho 61 37803 sho 62 104810 sho 62 104811 sho 62 104812 sho 62 104813 sho 63 54405 etc as to the polyvalent carboxylic acid ester used in the above solid catalyst component a its concrete examples are esters of phthalic acid maleic acid substituted malonic acid etc with an alcohol having two or more carbon atoms as to the mg compound used in the above a in the present invention various ones are mentioned and mg compound having a reducing capability.