DESCRIPTION SOLID TITANIUM CATALYST COMPONENT, OLEFIN POLYMERIZATION CATALYST, AND OLEFIN POLYMERIZATION PROCESS TECHNICAL FIELD The present invention relates to a solid titanium catalyst component which is preferably used for olefin polymerization, particularly α-olefins polymerization. The invention also relates to an olefin polymerization catalyst containing the solid titanium catalyst component. Further, the invention also relates to an olefin polymerization process using the olefin polymerization catalyst. BACKGROUND ART As catalysts used for producing olefin polymers such as a homopolymer of ethylene or an α-olefins and an ethylene/α-olefin copolymer, catalysts containing a titanium compound supported on a magnesium halide in an active state have been known in the past (the term "polymerization" is sometimes used to include copolymerization hereinafter). As such olefin polymerization catalysts, catalysts containing titanium tetrachloride or titanium trichloride, which are called Ziegler Natta catalysts, and catalysts comprising a solid titanium catalyst component consisting of magnesium, titanium, halogen and an electron donor and an organometallic compound are widely known. The latter catalysts exhibit high activity not only in polymerization of ethylene but also in polymerization of cc-olefins such as propylene and 1-butene. Further, the resulting α-olefins polymers sometimes have high stereoregularity. It has been reported in Japanese Patent Laid-Open Publication No. 63310/1982 (patent document 1) and the like that among such catalysts, a catalyst using a solid titanium catalyst component in which an electron donor selected from carboxylic acid esters (typical examples: phthalic acid esters) is supported, an aluminum alkyl compound as a co-catalyst component and a silicon compound having at least one Si-OR (wherein R is a hydrocarbon group) exhibits excellent polymerization activity and stereospecificity. The polymers obtained by the use of the above catalyst often have narrower molecular weight distribution as compared with polymers obtained by the use of Ziegler Natta catalyst. It is known that polymers having narrow molecular weight distribution tend to have "low melt flowability", "low melt tension", "inferior moldability", "slightly low rigidity", etc. On the other hand, from the viewpoints of increase of productivity, cost reduction, etc., various high-speed molding techniques, such as high-speed stretching technique having a purpose of increasing productivity of stretched films, have been developed. If such polymers having relatively narrow molecular weight distribution as above are intended to be stretched at high speed, neck-in or flapping of a film becomes conspicuous because of shortage of melt tension, and increase of productivity sometimes becomes difficult. Therefore, polymers having higher melt tension have been desired in the market. In order to solve such problems, there have been made a large number of reports, such as reports on a method of preparing polymers of different molecular weights by multi-step polymerization to widen a molecular weight distribution of a polymer (Japanese Patent Laid-Open Publication No. 170843/1993 (patent document 2)), a catalyst containing plural kinds of electron donors (Japanese Patent Laid-Open Publication No. 7703/1991 (patent document 3)) and a catalyst using a succinic acid ester having asymmetric carbon as an electron donor contained in a solid titanium catalyst component (pamphlet of International Publication No. 01/057099 (patent document 4), pamphlet of International Publication No. 00/63261 (patent document 5), pamphlet of International Publication No. 02/30998 (patent document 6)). On the other hand, in Japanese Patent Laid-Open Publication No. 114811/2001 (patent document 7) and Japanese Patent Laid-Open Publication No. 40918/2003 (patent document 8), a solid catalyst component for polymerization of olefin(s), which is obtained by bringing a titanium compound, a magnesium compound and an electron donating compound into contact with one another, and a catalyst for polymerization of olefin(s), which contains this catalyst component, are disclosed. As this electron donating compound, a 1,2-cyclohexanedicarboxylic acid ester having a trans purity of not less than 80% is used in the invention described in the patent document 7, and a cyclohexenedicarboxylic acid diester is used in the invention described in the patent document 8. As an example of this cyclohexenedicarboxylic acid diester, only a 1-cyclohexenedicarboxylic acid diester wherein an alkoxycarbonyl group is bonded to the first position and the second position of a cyclohexene ring of 1-cyclohexene is disclosed (paragraphs "0021" to "0024", and working examples). In the patent documents 7 and 8, however, there is no description of molecular weight distribution of an olefin polymer. The present applicant has disclosed in a pamphlet of International Publication No. 2006/077945 that a solid titanium catalyst component containing a specific cyclic ester compound as an electron donor component gives an olefin polymer having an extremely wide molecular weight distribution (patent document 9). Patent document 1: Japanese Patent Laid-Open Publication No. 63310/1982 Patent document 2: Japanese Patent Laid-Open Publication No. 170843/1993 Patent document 3: Japanese Patent Laid-Open Publication No. 7703/1991 Patent document 4: pamphlet of International Publication No. 01/057099 Patent document 5: pamphlet of International Publication No. 00/63261 Patent document 6: pamphlet of International Publication No. 02/30998 Patent document 7: Japanese Patent Laid-Open Publication No. 114811/2001 Patent document 8: Japanese Patent Laid-Open Publication No. 40918/2003 Patent document 9: pamphlet of International Publication No. 2006/077945 DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION According to the studies by the present inventors, the catalysts of the patent documents 1 to 8 are insufficient in the effect of widening a molecular weight distribution of an olefin polymer, or they are catalysts that widen a molecular weight distribution by increasing a low-molecular weight component. On the other hand, there is estimation in the market that these catalysts cannot be said to be satisfactory in improvement in melt tension of an olefin polymer, and from the viewpoint of cost reduction, development of a catalyst capable of producing an olefin polymer having a wider molecular weight distribution through a simpler process has been desired in the market. The cyclic ester compound contained in the solid titanium catalyst component described in the patent document 9 is often a compound more expensive than electron donors contained in conventional solid titanium catalyst components. On that account, the solid titanium catalyst component disclosed in the patent document 9 has high production cost though it exhibits high performance in point of widening a molecular weight distribution, and therefore, improvement in production cost has been desired. Accordingly, it is an object of the present invention to provide a catalyst component and a catalyst which are capable of simply and easily producing an olefin polymer which has a wide molecular weight distribution, high stereoregularity and high melt tension and is suitable for high-speed stretching and high-speed molding, at a cost equivalent to that of conventional polymers. MEANS TO SOLVE THE PROBLEM The present inventors have earnestly studied, and as a result, they have found that when a solid titanium catalyst component containing plural kinds of specific cyclic ester compounds having plural carboxylic acid ester groups is used, (1) an olefin polymer having a wide molecular weight distribution can be prepared, and (2) the electron donor exerts an effect as a stereoregularity controlling agent, and an improving effect relating to control of streoregularity can be obtained though such an improving effect is not exerted in the case of using a cyclic ester compound singly. Thus, the present inventors have achieved the present invention. In any of the patent documents 7 and 8, the cyclic ester compound (a) represented by the following formula (1) and having a substituent R is neither described nor suggested. The solid titanium catalyst component (I) of the present invention comprises titanium, magnesium, halogen, a cyclic ester compound (a) represented by the following formula (1) and a cyclic ester compound (b) represented by the following formula (2). (Formula Removed) In the formula (1), n is an integer of 5 to 10. R2 and R3 are each independently COOR1 or R, and at least one of R2 and R3 is COOR1. A single bond (except Ca-Ca bond, and Ca-Cb bond in the case where R3 is R) in the cyclic skeleton may be replaced with a double bond. R1 is each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms. Plural R are each independently an atom or a group selected from a hydrogen atom, a hydrocarbon group of 1 to 20 carbon atoms, a halogen atom, a nitrogen-containing group, an oxygen-containing group, a phosphorous-containing group, a halogen-containing group and a silicon-containing group, and they may be bonded to one another to from a ring, but at least one R is not a hydrogen atom. In a skeleton of the ring formed by bonding of plural R to one another may be contained a double bond, and when two or more Ca to each of which COOR1 is bonded are contained in the skeleton of the ring, the number of carbon atoms to constitute the skeleton of the ring is 5 to 10. (Formula Removed) In the formula (2), n is an integer of 5 to 10. R4 and R5 are each independently COOR1 or a hydrogen atom, at least one of R4 and R5 is COOR1, R1 is each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms, and a single bond (except Ca-Ca bond, and Ca-Cb bond in the case where R5 is R) in the cyclic skeleton may be replaced with a double bond. In the formula (1), all the bonds between carbon atoms in the cyclic skeleton are preferably single bonds. In the formula (1), n is preferably 6. The cyclic ester compound (a) is preferably a compound represented by the following formula (la). (Formula Removed) In the formula (la), n is an integer of 5 to 10. A single bond (except Ca-Ca bond and Ca-Cb bond) in the cyclic skeleton may be replaced with a double bond. R1 is each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms. Plural R are each independently an atom or a group selected from a hydrogen atom, a hydrocarbon group of 1 to 20 carbon atoms, a halogen atom, a nitrogen-containing group, an oxygen-containing group, a phosphorous-containing group, a halogen-containing group and a silicon-containing group, and they may be bonded to one another to from a ring, but at least one R is not a hydrogen atom. In a skeleton of the ring formed by bonding of plural R to one another may be contained a double bond, and when two or more Ca to each of which COOR1 is bonded are contained in the skeleton of the ring, the number of carbon atoms to constitute the skeleton of the ring is 5 to 10. In the formula (2), all the bonds between carbon atoms in the cyclic skeleton are preferably single bonds. In the formula (2), n is preferably 6. The cyclic ester compound (b) is preferably a compound represented by the following formula (2a). (Formula Removed) In the formula (2a), n is an integer of 5 to 10. R1 is each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms. A single bond (except Ca-Ca bond and Ca-Cb bond) in the cyclic skeleton may be replaced with a double bond. The olefin polymerization catalyst of the present invention comprises: the above-mentioned solid titanium catalyst component (I), and an organometallic compound catalyst component (II) containing a metallic element selected from the group 1, the group 2 and the group 13 of the periodic table. The olefin polymerization catalyst of the invention may further comprise an electron donor (III). The process for preparing an olefin polymer of the present invention comprises polymerizing an olefin in the presence of the above-mentioned olefin polymerization catalyst. EFFECT OF THE INVENTION The solid titanium catalyst component, the olefin polymerization catalyst and the process for preparing an olefin polymer according to the invention are suitable for preparing an olefin polymer having a wide molecular weight distribution with high activity. If the solid titanium catalyst component, the olefin polymerization catalyst and the process for preparing an olefin polymer according to the invention are used, it can be expected that preparation of an olefin polymer excellent not only in molding properties such as high-speed stretchability and high-speed moldability but also in rigidity becomes possible. The cyclic ester compound (a) is often a compound more expensive than electron donors contained in the conventional solid titanium catalyst components. On the other hand, the cyclic ester compound (b) is often a compound having a price of not more than 1/10 of the price of the cyclic ester compound (a). On that account, the solid titanium catalyst component of the invention capable of keeping the effect of widening a molecular weight distribution with decreasing the content of the cyclic ester compound (a) has an effect of reducing the production cost. Further, if the solid titanium catalyst component containing plural kinds of specific cyclic ester compounds having plural carboxylic acid ester groups is used, the electron donor (III) exerts an effect as a stereoregularity controlling agent, and an improving effect relating to control of stereoregularity can be obtained though such an effect is not exerted in the case of using a cyclic ester compound singly. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph showing a relationship between a proportion of DMCHIBU added and an Mw/Mn value. BEST MODE FOR CARRYING OUT THE INVENTION The solid titanium catalyst component (I), the olefin polymerization catalyst and the process for preparing an olefin polymer according to the invention are described in detail hereinafter. Solid titanium catalyst component (I) The solid titanium catalyst component (I) of the invention comprises titanium, magnesium, halogen, a cyclic ester compound (a) and a cyclic ester compound (b). Cyclic ester compound (a) The cyclic ester compound (a) has plural carboxylic acid ester groups and is represented by the following formula (1). (Formula Removed) In the formula (1), n is an integer of 5 to 10, preferably an integer of 5 to 7, particularly preferably 6. Ca and Cb are each a carbon atom. R2 and R3 are each independently COOR1 or R, and at least one of R2 and R3 is COOR1. Although all the bonds between carbon atoms in the cyclic skeleton are preferably single bonds, any one of the single bonds other than Ca-Ca bond and Ca-Cb bond in the case where R3 is R in the cyclic skeleton may be replaced with a double bond. Plural R1 are each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 2 to 8 carbon atoms, still more preferably 4 to 8 carbon atoms, particularly preferably 4 to 6 carbon atoms. Examples of the hydrocarbon groups include ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, hexyl group, heptyl group, octyl group, 2-ethylhexyl group, decyl group, dodecyl group, tetradecyl group, hexadecyl group, octadecyl group and eicosyl group. Of these, n-butyl group, isobutyl group, hexyl group and octyl group are preferable, and n-butyl group and isobutyl group are particularly preferable, because an olefin polymer having a wide molecular weight distribution is apt to be prepared. Plural R are each independently an atom or a group selected from a hydrogen atom, a hydrocarbon group of 1 to 20 carbon atoms, a halogen atom, a nitrogen-containing group, an oxygen-containing group, a phosphorous-containing group, a halogen-containing group and a silicon-containing group, but at least one R is not a hydrogen atom. Of the above groups, a hydrocarbon group of 1 to 20 carbon atoms is preferable as R other than a hydrogen atom. Examples of the hydrocarbon groups of 1 to 20 carbon atoms include aliphatic hydrocarbon groups, alicyclic hydrocarbon groups and aromatic hydrocarbon groups, such as methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group, n-pentyl group, cyclopentyl group, n-hexyl group, cyclohexyl group, vinyl group, phenyl group and octyl group. Of these, aliphatic hydrocarbon groups are preferable, and specifically, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group and sec-butyl group are preferable. Plural R may be bonded to one another to form a ring, and in a skeleton of the ring formed by bonding of plural R to one another, a double bond may be contained. When two or more Ca to each of which COOR1 is bonded are contained in the skeleton of the ring, the number of carbon atoms to constitute the skeleton of the ring is 5 to 10. Examples of the skeletons of the ring include norbornane skeleton and tetracyclododecene skeleton. Plural R may be carbonyl structure-containing groups, such as carboxylic acid ester group, alkoxy group, siloxy group, aldehyde group and acetyl group, and their substituents preferably contain one or more hydrocarbon groups. Examples of such cyclic ester compounds (a) include the following compounds which are described in a pamphlet of International Publication No. 2006/077945 (patent document 9) : diethyl 3-methylcyclohexane-l,2-dicarboxylate, di-n-propyl 3-methylcyclohexane-l,2-dicarboxylate, diisopropyl 3-methylcyclohexane-l,2-dicarboxylate, di-n-butyl 3-methylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methylcyclohexane-l,2-dicarboxylate, dihexyl 3-methylcyclohexane-l,2-dicarboxylate, diheptyl 3-methylcyclohexane-l,2-dicarboxylate, dioctyl 3-methylcyclohexane-l,2-dicarboxylate, di-2-ethylhexyl 3-methylcyclohexane-l,2-dicarboxylate, didecyl 3-methylcyclohexane-l,2-dicarboxylate, diethyl 4-methylcyclohexane-l,3-dicarboxylate, diisobutyl 4-methylcyclohexane-l,3-dicarboxylate, diethyl 4-methylcyclohexane-l,2-dicarboxylate, di-n-propyl 4-methylcyclohexane-l,2-dicarboxylate, diisopropyl 4-methylcyclohexane-l,2-dicarboxylate, di-n-butyl 4-methylcyclohexane-l,2-dicarboxylate, diisobutyl 4-methylcyclohexane-l,2-dicarboxylate, dihexyl 4-methylcyclohexane-l,2-dicarboxylate, diheptyl 4-methylcyclohexane-l,2-dicarboxylate, dioctyl 4-methylcyclohexane-l,2-dicarboxylate, di-2-ethylhexyl 4-methylcyclohexane-l,2-dicarboxylate, didecyl 4-methylcyclohexane-l,2-dicarboxylate, diethyl 5-methylcyclohexane-l,3-dicarboxylate, diisobutyl 5-methylcyclohexane-l,3-dicarboxylate, diethyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, di-n-propyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, diisopropyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, di-n-butyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, diisobutyl 3, 4-dimethylcyclohexane-l,2-dicarboxylate, dihexyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, diheptyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, dioctyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, di-2-ethylhexyl 3,4-dimethylcyclohexane-l,2- dicarboxylate, didecyl 3,4-dimethylcyclohexane-l,2-dicarboxylate, diethyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-n-propyl 3, 6-dimethylcyclohexane-l,2-dicarboxylate, diisopropyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-n-butyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, diisobutyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, dihexyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, diheptyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, dioctyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-2-ethylhexyl 3,6-dimethylcyclohexane-l, 2- dicarboxylate, didecyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, diethyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, di-n-propyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, diisopropyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, di-n-butyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, diisobutyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, dihexyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, dioctyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, didecyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, diethyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, di-n-propyl 3-methyl-6-ethylcyclohexane-l,2- dicarboxylate, diisopropyl 3-methyl-6-ethylcyclohexane-1,2- dicarboxylate, di-n-butyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, dihexyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, diheptyl 3-methyl~6-ethylcyclohexane-l,2-dicarboxylate, dioctyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, di-2-ethylhexyl 3-methyl-6-ethylcyclohexane-l,2- dicarboxylate, didecyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, diethyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, di-n-propyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, diisopropyl 3-methyl~6-n-propylcyclohexane-l,2-dicarboxylate, di-n-butyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, dihexyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, diheptyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, dioctyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, di-2-ethylhexyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, didecyl 3-n\ethyl-6-n-propylcyclohexane-l, 2-dicarboxylate, diethyl 3-hexylcyclohexane-l,2-dicarboxylate, diisobutyl 3-hexylcyclohexane-i,2-dicarboxylate, diethyl 3,6-dihexylcyclohexane-l,2-dicarboxylate, diisobutyl 3-hexyl-6-pentylcyclohexane-l,2-dicarboxylate, diethyl 3-methylcyclopentane-l,2-dicarboxylatef diisobutyl 3-methylcyclopentane-l,2-dicarboxylate, diheptyl 3-methylcyclopentane-l,2-dicarboxylate, didecyl 3-methylcyclopentane-l,2-dicarboxylate, diethyl 4-methylcyclopentane-l,3-dicarboxylate, diisobutyl 4-methylcyclopentane-l,3-dicarboxylate, diethyl 4-methylcyclopentane-l,2-dicarboxylate, diisobutyl 4-methylcyclopentane-l,2-dicarboxylate, diheptyl 4-methylcyclopentane-l,2-dicarboxylate, didecyl 4-methylcyclopentane-l,2-dicarboxylate, diethyl 5-methylcyclopentane-l,3-dicarboxylate, diisobutyl 5-methylcyclopentane-l,3-dicarboxylate, diethyl 3,4-dimethylcyclopentane-l,2-dicarboxylate, diisobutyl 3,4-dimethylcyclopentane-l,2-dicarboxylate, diheptyl 3,4-dimethylcyclopentane-l,2-dicarboxylate, didecyl 3,4-dimethylcyclopentane-l,2-dicarboxylate, diethyl 3,5-dimethylcyclopentane-l,2-dicarboxylate, diisobutyl 3,5-dimethylcyclopentane-l,2-dicarboxylate, diheptyl 3,5-dimethylcyclopentane-l,2-dicarboxylate, didecyl 3,5-dimethylcyclopentane-l,2-dicarboxylate, diethyl 3-hexylcyclopentane-l,2-dicarboxylate, diethyl 3,5-dihexylcyclopentane-l,2-dicarboxylate, diisobutyl 3-hexyl-5-pentylcyclopentane-l,2-dicarboxylate, diethyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, di-n-propyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, diisopropyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, di-n-butyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, diisobutyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, dihexyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, dioctyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, didecyl 3-methyl-5-n-propylcyclopentane-lf 2- dicarboxylate, diethyl 3-methylcycloheptane-l,2-dicarboxylate, diisobutyl 3-methylcycloheptane-l,2-dicarboxylate, diheptyl 3-methylcycloheptane-l,2-dicarboxylate, didecyl 3-methylcycloheptane-l,2-dicarboxylate, diethyl 4-methylcycloheptane-l,3-dicarboxylate, diisobutyl 4-methylcycloheptane-l,3-dicarboxylate, diethyl 4-methylcycloheptane-l,2-dicarboxylate, diisobutyl 4-methylcycloheptane-l,2-dicarboxylate, diheptyl 4-methylcycloheptane-l,2-dicarboxylate, didecyl 4-methylcycloheptane-l,2-dicarboxylate, diethyl 5-methylcycloheptane-l,3-dicarboxylate, diisobutyl 5-methylcycloheptane-l,3-dicarboxylate, diethyl 3,4-dimethylcycloheptane-l,2-dicarboxylate, diisobutyl 3,4-dimethylcycloheptane-l,2-dicarboxylate, diheptyl 3,4-dimethylcycloheptane-l,2-dicarboxylate, didecyl 3,4-dimethylcycloheptane-l,2-dicarboxylate, diethyl 3,7-dimethylcycloheptane-l,2-dicarboxylate, diisobutyl 3,7-dimethylcycloheptane-l,2-dicarboxylate, diheptyl 3,7-dimethylcycloheptane-l,2-dicarboxylate, didecyl 3,7-dimethylcycloheptane-l,2-dicarboxylate, diethyl 3-hexylcycloheptane-l,2-dicarboxylate, diethyl 3,7-dihexylcycloheptane-l,2-dicarboxylate, diisobutyl 3-hexyl-7-pentylcycloheptane-l,2- dicarboxylate, diethyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, di-n-propyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, diisopropyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, di-n-butyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, diisobutyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, dihexyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, dioctyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, didecyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, diethyl 3-methylcyclooctane-l,2-dicarboxylate, diethyl 3-methylcyclodecane-l,2-dicarboxylate, diisobutyl 3-vinylcyclohexane-l,2-dicarboxylate, diisobutyl 3,6-diphenylcyclohexane-l,2-dicarboxylate, diethyl 3,6-dicyclohexylcyclohexane-l,2-dicarboxylate, diisobutyl norbornane-2,3-dicarboxylate, diisobutyl tetracyclododecane-2,3-dicarboxylate, diethyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, di-n-propyl 3,6-dimethyl-4-cyclohexene-l, 2-dicarboxylate, diisopropyl 3,6-dimethyl-4-cyclohexene-l, 2-dicarboxylate, di-n-butyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, diisobutyl 3,6-dimethyl-4-cyclohexene-l, 2-dicarboxylate, dihexyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, diheptyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, dioctyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, di-2-ethylhexyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, didecyl 3,6-dimethyl-4-cyclohexene-l,2-dicarboxylate, diethyl 3,6-dihexyl-4-cyclohexene-l,2-dicarboxylate, and diisobutyl 3-hexyl-6-pentyl-4-cyclohexene-l,2-dicarboxylate. Further, dicarboxylic acid esters of cyclic diol compounds corresponding to the above compounds can be also mentioned as preferred compounds. Preferred examples of such compounds include: 3,6-dimethylcyclohexyl-l,2-diacetate, 3,6-dimethylcyclohexyl-l,2-dibutanate, 3-methyl-6-propylcyclohexyl-l,2-diolacetate, 3-methyl-6-propylcyclohexyl-l,2-butanate, 3,6-dimethylcyclohexyl-l,2-dibenzoate, 3,6-dimethylcyclohexyl-l,2-ditoluate, 3-methyl-6-propylcyclohexyl-l,2-dibenzoate, and 3-methyl-6-propylcyclohexyl-l,2-ditoluate. In such compounds having diester structure as above, isomers such as cis form and trans form derived from plural COOR1 groups in the formula (1) are present, and any structure has an effect which is in accord with the object of the invention. However, a compound having a higher content of trans form is preferable. In the case of a compound having a higher content of trans form, not only an effect of widening a molecular weight distribution but also activity and stereoregularity of the resulting polymer tend to become higher. As the cyclic ester compounds (a), compounds represented by the following formulas (1-1) to (1-6) are preferable. In the formulas (1-1) to (1-6), R1 and R are the same as those previously described. In the formulas (1-1) to (1-3), a single bond (except Ca-Ca bond and Ca-Cb bond) in the cyclic skeleton may be replaced with a double bond. In the formulas (1-4) to (1-6), a single bond (except Ca-Ca bond) in the cyclic skeleton may be replaced with a double bond. In the formulas (1-3) and (1-6), n is an integer of 7 to 10. As the cyclic ester compound (a), a compound represented by the following formula (la) is particularly preferable. (Formula Removed) In the formula (la), n, R1 and R are the same as those previously described (that is, they have the same meanings as those in the formula (1)), and a single bond (except Ca-Ca bond and Ca-Cb bond) in the cyclic skeleton may be replaced with a double bond. Examples of the compounds represented by the above formula (la) include: diisobutyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-n-octyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, di-n-octyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3-methyl-6-n-propylcyclohexane-l,2- dicarboxylate, di-n-octyl 3-methyl-6-n-propylcyclohexane-l,2- dicarboxylate, diisobutyl 3, 6-diethylcyclohexane-l, 2-dicarboxylate, di-n-hexyl 3,6-diethylcyclohexane-l,2-dicarboxylate, di-n-octyl 3,6-diethylcyclohexane-l,2-dicarboxylate, diisobutyl 3,5-dimethylcyclopentane-l,2-dicarboxylate, di-n-hexyl 3,5-dimethylcyclopentane-l,2-dicarboxylate, di-n-octyl 3, 5-dimethylcyclopentane-l,2-dicarboxylate, diisobutyl 3-methyl-5-ethylcyclopentane-l,2- dicarboxylate, di-n-hexyl 3-methyl-5-ethylcyclopentane-l,2- dicarboxylate, di-n-octyl 3-methyl-5-ethylcyclopentane-l,2- dicarboxylate, di-n-hexyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, di-n-octyl 3-methyl-5-n-propylcyclopentane-l,2- dicarboxylate, diisobutyl 3,5-diethylcyclopentane-l,2-dicarboxylate, di-n-hexyl 3,5-diethylcyclopentane-l,2-dicarboxylate, di-n-octyl 3,5-diethylcyclopentane-l,2-dicarboxylate, diisobutyl 3,7-dimethylcycloheptane-l,2-dicarboxylate, di-n-hexyl 3, 7-dimethylcycloheptane-l,2-dicarboxylate, di-n-octyl 3, 7-dimethylcycloheptane-l,2-dicarboxylate, diisobutyl 3-methyl-7-ethylcycloheptane-l,2- dicarboxylate, di-n-hexyl 3-methyl-7-ethylcycloheptane-l,2- dicarboxylate, di-n-octyl 3-methyl-7-ethylcycloheptane-l,2- dicarboxylate, di-n-hexyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, di-n-octyl 3-methyl-7-n-propylcycloheptane-l,2- dicarboxylate, diisobutyl 3,7-diethylcycloheptane-l,2-dicarboxylate, di-n-hexyl 3,7-diethylcycloheptane-l,2-dicarboxylate, and di-n-octyl 3,7-diethylcycloheptane-l,2-dicarboxylate. [0057] Of the above compounds, more preferable are: diisobutyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, di-n-octyl 3,6-dimethylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, di-n-octyl 3-methyl-6-ethylcyclohexane-l,2-dicarboxylate, diisobutyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, di-n-octyl 3-methyl-6-n-propylcyclohexane-l,2-dicarboxylate, diisobutyl 3,6-diethylcyclohexane-l,2-dicarboxylate, di-n-hexyl 3,6-diethylcyclohexane-l,2-dicarboxylate, and di-n-octyl 3,6-diethylcyclohexane-l,2-dicarboxylate. Although these compounds can be produced by the use of Diels-Alder reaction, polyene compounds as raw materials are relatively expensive, and therefore, the production cost of the above compounds tends to become a little higher than that of the conventional electron donor compounds. In such cyclic ester compounds (a) having diester structure as above, isomers such as cis form and trans form are present, and any structure has an effect which is in accord with the object of the invention. However, a compound having a higher content of trans form is preferable. In the case of a compound having a higher content of trans form, not only an effect of widening a molecular weight distribution but also activity and stereoregularity of the resulting polymer tend to become higher. The proportion of the trans form in the total of the cis form and the trans form is preferably not less than 51%. The lower limit is more preferably 55%, still more preferably 60%, particularly preferably 65%. On the other hand, the upper limit is preferably 100%, more preferably 90%, still more preferably 85%, particularly preferably 79%. Cyclic ester compound (b) The cyclic ester compound (b) has plural carboxylic acid ester groups and is represented by the following formula (2). (Formula Removed) In the formula (2), n is an integer of 5 to 10, preferably an integer of 5 to 7, particularly preferably 6. Ca and Cb are each a carbon atom. Although all the bonds between carbon atoms in the cyclic skeleton are preferably single bonds, any one of the single bonds other than Ca-Ca bond and Ca-Cb bond in the case where R5 is a hydrogen atom in the cyclic skeleton may be replaced with a double bond. R4 and R5 are each independently COOR1 or a hydrogen atom, at least one of R4 and R5 is COOR1, and R1 is each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms. Plural R1 are each independently a monovalent hydrocarbon group of 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 2 to 8 carbon atoms, still more preferably 4 to 8 carbon atoms, particularly preferably 4 to 6 carbon atoms. Examples of the hydrocarbon groups include ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, hexyl group, heptyl group, octyl group, 2-ethylhexyl group, decyl group, dodecyl group, tetradecyl group, hexadecyl group, octadecyl group and eicosyl group. Of these, n-butyl group, isobutyl group, hexyl group and octyl group are preferable, and n-butyl group and isobutyl group are particularly preferable, because an olefin polymer having a wide molecular weight distribution is apt to be prepared. Examples of such cyclic ester compounds (b) include: diethyl cyclohexane-1,2-dicarboxylate, di-n-propyl cyclohexane-1,2-dicarboxylate, diisopropyl cyclohexane-1,2-dicarboxylate, di-n-butyl cyclohexane-1,2-dicarboxylate, diisobutyl cyclohexane-1,2-dicarboxylate, dihexyl cyclohexane-1,2-dicarboxylate, diheptyl cyclohexane-1,2-dicarboxylate, dioctyl cyclohexane-1,2-dicarboxylate, di-2-ethylhexyl cyclohexane-1,2-dicarboxylate, didecyl cyclohexane-1,2-dicarboxylate, diethyl cyclohexane-1,3-dicarboxylate, diisobutyl cyclohexane-1,3-dicarboxylate, diethyl cyclopentane-1,2-dicarboxylate, diisopropyl cyclopentane-1,2-dicarboxylate, diisobutyl cyclopentane-1,2-dicarboxylate, diheptyl cyclopentane-1,2-dicarboxylate, didecyl cyclopentane-1,2-dicarboxylate, diethyl cyclopentane-1,3-dicarboxylate, diisobutyl cyclopentane-1,3-dicarboxylate, diethyl cycloheptane-1,2-dicarboxylate, diisopropyl cycloheptane-1,2-dicarboxylate, diisobutyl cycloheptane-1,2-dicarboxylate, diheptyl cycloheptane-1,2-dicarboxylate, didecyl cycloheptane-1,2-dicarboxylate, diethyl cycloheptane-1,3-dicarboxylate, diisobutyl cycloheptane-1,3-dicarboxylate, diethyl cyclooctane-1,2-dicarboxylate, diethyl cyclodecane-1, 2-dicarboxylate, diethyl 4-cyclohexene-l, 2-dicarboxylate, di-n-propyl 4-cyclohexene-l,2-dicarboxylate, diisopropyl 4-cyclohexene-l,2-dicarboxylate, di-n-butyl 4-cyclohexene-l,2-dicarboxylate, diisobutyl 4-cyclohexene-l,2-dicarboxylate, dihexyl 4-cyclohexene-l, 2-dicarboxylate, diheptyl 4-cyclohexene-l, 2-dicarboxylate, dioctyl 4-cyclohexene-l, 2-dicarboxylate, didecyl 4-cyclohexene-l,2-dicarboxylate, diethyl 4-cyclohexene-l,3-dicarboxylate, diisobutyl 4-cyclohexene-l,3-dicarboxylate, diethyl 3-cyclopentene-l, 2-dicarboxylate, diisopropyl 3-cyclopentene-l,2-dicarboxylate, diisobutyl 3-cyclopentene-l,2-dicarboxylate, diheptyl 3-cyclopentene-l, 2-dicarboxylate, didecyl 3-cyclopentene-l,2-dicarboxylate, diethyl 3-cyclopentene-l,3-dicarboxylate, diisobutyl 3-cyclopentene-l,3-dicarboxylate, diethyl 4-cycloheptene-l,2-dicarboxylate, diisopropyl 4-cycloheptene-l,2-dicarboxylate, diisobutyl 4-cycloheptene-l,2-dicarboxylate, diheptyl 4-cycloheptene-l,2-dicarboxylate, didecyl 4-cycloheptene-l,2-dicarboxylate, diethyl 4-cycloheptene-l, 3-dicarboxylate, diisobutyl 4-cycloheptene-l,3-dicarboxylate, diethyl 5-cyclooctene-l, 2-dicarboxylate, and diethyl 6-cyclodecene-l,2-dicarboxylate. Further, dicarboxylic acid esters of cyclic diol compounds corresponding to the above compounds can be also mentioned as preferred compounds. Examples of such compounds include: cyclohexyl-1,2-diacetate, cyclohexyl-1,2-dibutanate, cyclohexyl-1,2-dibenzoate, and cyclohexyl-1,2-ditoluate. In such compounds having diester structure as above, isomers such as cis form and trans form are present, and any structure has an effect which is in accord with the object of the invention. The proportion of the trans form in the total of the cis form and the trans form is preferably not less than 51%. The lower limit is more preferably 55%, still more preferably 60%, particularly preferably 65%. On the other hand, the upper limit is preferably 100%, more preferably 90%, still more preferably 85%, particularly preferably 79%. Although the reason is not clear, it is presumed that variations of the later-described stereoisomers are within the region suitable for widening the molecular weight distribution. In particular, the cyclohexane-1,2-dicarboxylic acid diester wherein n in the formula (2) is 6 has a trans purity of the above range. If the trans purity is less than 51%, the effect of widening molecular weight distribution, activity, stereospecificity, etc. sometimes become insufficient. If the trans purity exceeds 79%, the effect of widening molecular weight distribution sometimes becomes insufficient. That is to say, when the trans purity is in the above range, there are many advantages in making the effect of widening molecular weight distribution of the resulting polymer and the activity of catalyst or the high stereoregularity of the resulting polymer compatible with each other to a high level. As the cyclic ester compounds (b), compounds having cycloalkane-1,2-dicarboxylic acid diester structure and represented by the following formula (2a) are preferable, and particularly preferable are: di-n-butyl cyclohexane-1,2-dicarboxylate, diisobutyl cyclohexane-1,2-dicarboxylate, dihexyl cyclohexane-1,2-dicarboxylate, diheptyl cyclohexane-1,2-dicarboxylate, dioctyl cyclohexane-1,2-dicarboxylate, di-2-ethylhexyl cyclohexane-1,2-dicarboxylate, diisobutyl cyclopentane-1,2-dicarboxylate, diheptyl cyclopentane-1,2-dicarboxylate, diisobutyl cycloheptane-1,2-dicarboxylate, diheptyl cycloheptane-1,2-dicarboxylate, etc. (Formula Removed) In the formula (2a), R1 is the same as that previously described (that is, it has the same meaning as that in the formula (2)), and a single bond (except Ca-Ca bond and Ca-Cb bond) in the cyclic skeleton may be replaced with a double bond. Of the above compounds, more preferable are: diisobutyl cyclohexane-1,2-dicarboxylate, dihexyl cyclohexane-1,2-dicarboxylate, diheptyl cyclohexane-1,2-dicarboxylate, dioctyl cyclohexane-1,2-dicarboxylate, and di-2-ethylhexyl cyclohexane-1,2-dicarboxylate. The reason is that not only the catalytic performance is excellent but also these compounds can be prepared relatively inexpensively by utilizing the Diels-Alder reaction. These compounds may be used singly, or may be used in combination of two or more kinds. Further, the cyclic ester compounds (a) and (b) may be used in combination with the later-described catalyst component (c), within limits not detrimental to the object of the present invention. The combining molar ratio of the cyclic ester compound (a) to the cyclic ester compound (b) (cyclic ester compound (a)/(cyclic ester compound (a)+cyclic ester compound (b))xlOO (% by mol)) is preferably not less than 10% by mol. The combining molar ratio is more preferably not less than 30% by mol, still more preferably not less than 40% by mol, particularly preferably not less than 50% by mol. The upper limit is preferably 99% by mol, more preferably 90% by mol, still more preferably 85% by mol, particularly preferably 80% by mol. The cyclic ester compounds (a) and (b) may be formed during the course of preparation of the solid titanium catalyst component (I). For example, by providing a step of substantially bringing carboxylic anhydrides or carboxylic dihalides corresponding to the cyclic ester compounds (a) and (b) into contact with the corresponding alcohols, the cyclic ester compounds (a) and (b) can be incorporated in the solid titanium catalyst component. By the process for preparing an olefin polymer of the invention, a polymer having a wide molecular weight distribution is obtained. Although the reason is not clear, such a cause as described below is presumed. The cyclic hydrocarbon structure is known to form various stereostructures such as chair form and boat foam. Moreover, if the cyclic structure has a substituent, the variation of stereostructure which can be taken is further increased. Furthermore, if the bond between a carbon atom to which the ester group (COOR1 group) is bonded and another carbon atom to which the ester group (COOR1 group) is bonded is a single bond, said carbon atoms being among the carbon atoms to constitute the cyclic skeleton of the cyclic ester compound, the variation of stereostructure which can be taken is widened. Such various stereostructures which can be taken lead to formation of various active sites on the solid titanium catalyst component (I). As a result, when olefin polymerization is carried out using the solid titanium catalyst component (I), olefin polymers having various molecular weights can be prepared at once. That is to say, an olefin polymer having a wide molecular weight distribution can be prepared. Under the conditions of the combining molar ratio of the cyclic ester compound (a) in a wide range, that is, even if the content of the cyclic ester compound (a) in the solid titanium catalyst component is low, the solid titanium catalyst component (I) of the invention can give an olefin polymer having an extremely wide molecular weight distribution. Although the reason of this effect is not clear, the present inventors have presumed as follows. It is obvious that owing to the presence of the substituent R, the cyclic ester compound (a) has an extremely larger number of variations of stereostructures which can be formed, as compared with the cyclic ester compound (b). On this account, it is thought that the influence of the cyclic ester compound (a) on the molecular weight distribution becomes dominant, and even if the combining molar ratio is low, the cyclic ester compound (a) can give an olefin polymer having an extremely wide molecular weight distribution. On the other hand, the cyclic ester compound (a) and the cyclic ester compound (b) are relatively analogous in structure, and therefore, these compounds hardly have influence on each other with regard to their basic properties such as activity and stereoregularity. (If compounds of different structures are used, activity, stereoregularity or the like often changes violently, or the effect of one compound often becomes dominant.) On this account, even if the content of the cyclic ester compound (a) is low, the solid titanium catalyst component (I) of the invention can give an olefin polymer having an extremely wide molecular weight distribution and high streoregularity with high activity. In the preparation of the solid titanium catalyst component (I) of the invention, a magnesium compound and a titanium compound are used in addition to the above cyclic ester compounds (a) and (b). Magnesium compound Examples of the magnesium compounds include publicly known magnesium compounds, specifically, magnesium halides, such as magnesium chloride and magnesium bromide; magnesium alkoxyhalides, such as magnesium methoxychloride, magnesium ethoxychloride and magnesium phenoxychloride; alkoxymagnesiums, such as ethoxymagnesium, isopropoxymagnesium, butoxymagnesium and 2-ethylhexoxymagnesium: aryloxymagnesiums, such as phenoxymagnesium; and carboxylic acid salts of magnesium, such as magnesium stearate. These magnesium compounds may be used singly, or may be used in combination of two or more kinds. Further, these magnesium compounds may be complex compounds or double compounds with other metals, or mixtures with other metallic compounds. Of the above compounds, magnesium compounds containing halogen are preferable, and magnesium halides, particularly magnesium chloride, are preferably employed. Alkoxymagnesiums such as ethoxymagnesium are also preferably employed. The magnesium compound may be that derived from other substance, e.g., a magnesium compound obtained by bringing an organomagnesium compound such as Grignard reagent into contact with titanium halide, silicon halide, halogenated alcohol or the like. Titanium compound The titanium compound is, for example, a tetravalent titanium compound represented by the formula: (Formula Removed) wherein R is a hydrocarbon group, X is a halogen atom, and g is a number of 0 ≤ g ≤ 4. More specifically, there can be mentioned: titanium tetrahalides, such as TiCl4 and TiBr4; alkoxytitanium trihalides, such as Ti(OCH3)Cl3, Ti (0C2H5)C13, Ti(0-n-C4H9)Cl3, Ti(OC2H5)Br3 and Ti (0-isoC4H9) Br3; alkoxytitanium dihalides, such as Ti(OCH3)2Cl2 and Ti(OC2H5)2Cl2; alkoxytitanium monohalides, such as Ti(OCH3) 3C1, Ti(0-n-C4H9)3C1 and Ti (OC2H5) 3Br; and tetraalkoxytitaniums, such as Ti(OCH3)4, Ti(OC2H5)4, Ti(OC4H9)4 and Ti(0-2-ethylhexyl)4. Of these, preferable are titanium tetrahalides, and particularly preferable is titanium tetrachloride. These titanium compounds may be used singly, or may be used in combination of two or more kinds. As such magnesium compounds and titanium compounds as above, compounds described in, for example, the patent document 1 and the patent document 2 in detail are also employable. For preparing the solid titanium catalyst component (I) of the invention, publicly known processes can be used without any restriction, except that the cyclic ester compounds (a) and (b) are used. Preferred examples of the processes include the following processes (P-l) to (P-4). (P-l) A process wherein a solid adduct consisting of a magnesium compound and a catalyst component (c), the cyclic ester compounds (a) and (b), and a titanium compound in a liquid state are brought into contact with one another in a suspension state in the presence of an inert hydrocarbon solvent. (P-2) A process wherein a solid adduct consisting of a magnesium compound and a catalyst component (c), the cyclic ester compounds (a) and (b) , and a titanium compound in a liquid state are brought into contact with one another plural times. (P-3) A process wherein a solid adduct consisting of a magnesium compound and a catalyst component (c), the cyclic ester compounds (a) and (b), and a titanium compound in a liquid state are brought into contact with one another plural times in a suspension state in the presence of an inert hydrocarbon solvent. (P-4) A process wherein a magnesium compound in a liquid state consisting of a magnesium compound and a catalyst component (c) , a titanium compound in a liquid state, and the cyclic ester compounds (a) and (b) are brought into contact with one another. The reaction temperature in the preparation of the solid titanium catalyst component (I) is in the range of preferably -30°C to 150°C, more preferably -25°C to 130°C, still more preferably -25°C to 120°C. The preparation of the solid titanium catalyst component can be carried out in the presence of a publicly known medium, when necessary. Examples of the media include aromatic hydrocarbons having slight polarity, such as toluene, and publicly known aliphatic hydrocarbons and alicyclic hydrocarbons, such as heptane, octane, decane and cyclohexane. Of these, aliphatic hydrocarbons are preferable. When olefin polymerization reaction is carried out using the solid titanium catalyst component (1) prepared under the conditions of the above range, the effect of obtaining a polymer having a high molecular weight distribution and the activity of catalyst or the high stereoregularity of the resulting polymer can be made compatible with each other to a high level. Catalyst component (c) As the catalyst component (c) used for forming the solid adduct or the magnesium compound in a liquid state, a publicly known compound capable of solubilizing the aforesaid magnesium compound in the temperature range of about room temperature to 300°C is preferable, and for example, alcohol, aldehyde, amine, carboxylic acid and mixtures thereof are preferable. As such compounds, compounds described in, for example, the patent document 1 and the patent document 2 in detail are also employable. Examples of the alcohols having ability to solubilize the magnesium compound include: aliphatic alcohols, such as methanol, ethanol, propanol, butanol, isobutanol, ethylene glycol, 2-methylpentanol, 2-ethylbutanol, n-heptanol, n-octanol, 2-ethylhexanol, decanol and dodecanol; alicyclic alcohols, such as cyclohexanol and methylcyclohexanol; aromatic alcohols, such as benzyl alcohol and methylbenzyl alcohol; and aliphatic alcohols having alkoxy group, such as n-butyl cellosolve. Examples of the carboxylic acids include organic carboxylic acids having 7 or more carbon atoms, such as caprylic acid and 2-ethylhexanoic acid. Examples of the aldehydes include aldehydes having 7 or more carbon atoms, such as capric aldehyde and 2-ethylhexyl aldehyde. Examples of the amines include amines having 6 or more carbon atoms, such as heptylamine, octylamine, nonylamine, laurylamine and 2-ethylhexylamine. As the catalyst components (c), the above alcohols are preferable, and ethanol, propanol, butanol, isobutanol, hexanol, 2-ethylhexanole, decanol, etc. are particularly preferable. Although the amounts of the magnesium compound and the catalyst component (c) used for preparing the solid adduct or the magnesium compound in a liquid state vary depending upon the types thereof, the contact conditions, etc., the magnesium compound is used in an amount of 0.1 to 20 mol/liter, preferably 0.5 to 5 mol/liter, based on the unit volume of the catalyst component (c). Further, a medium inert to the solid catalyst (c) is also employable in combination, when necessary. Preferred examples of the media include publicly known hydrocarbon compounds, such as heptane, octane and decane. The compositional ratio between magnesium in the resulting solid adduct or the magnesium compound in a liquid state and the catalyst component (c) varies depending upon the compounds used and cannot be defined indiscriminately, but the amount of the catalyst component (c) is preferably not less than 2.0 mol, more preferably not less than 2.2 mol, still more preferably not less than 2.3 mol, particularly preferably not less than 2.4 mol but not more than 5 mol, based on 1 mol of magnesium in the magnesium compound. Such cyclic ester compounds (a) and (b) and catalyst component (c) as above may be considered to belong to a component that is called an electron donor by a person skilled in the art. The electron donor component is known to exhibit an effect of enhancing stereoregularity of the resulting polymer, an effect of controlling a compositional distribution of the resulting copolymer, a coagulant effect of controlling particle shape or particle diameter of a catalyst particle, etc., with keeping high activity of the catalyst. It is thought that the cyclic ester compound (a) further exhibits an effect of controlling a molecular weight distribution because the cyclic ester compound (a) itself is an electron donor. In the solid titanium catalyst component (I) of the invention, the halogen/titanium ratio by atom (namely, number of moles of halogen atom/number of moles of titanium atom) is desired to be in the range of 2 to 100, preferably 4 to 90; the cyclic ester compound (a)/titanium ratio by mol (namely, number of moles of cyclic ester compound (a)/number of moles of titanium atom) and the cyclic ester compound (b)/titanium atom ratio by mol (namely, number of moles of cyclic ester compound (b)/number of moles of titanium atom) are each desired to be in the range of 0.01 to 100, preferably 0.2 to 10; and the catalyst component (c)/titanium atom ratio by mol is desired to be in the range of 0 to 100, preferably 0 to 10. With regard to a preferred ratio of the cyclic ester compound (a) to the cyclic ester compound (b), the lower limit of the value (% by mol) of 100 x cyclic ester compound (a)/(cyclic ester compound (a) + cyclic ester compound (b)) is 10% by mol, preferably 30% by mol, more preferably 40% by mol, particularly preferably 50% by mol, and the upper limit thereof is 99% by mol, preferably 90% by mol, more preferably 85% by mol, particularly preferably 80% by mol. The magnesium/titanium ratio by atom (namely, number of moles of magnesium atom/number of moles of titanium atom) is desired to be in the range of 2 to 100, preferably 4 to 50. The content of a component which may be contained in addition to the cyclic ester compounds (a) and (b), e.g., the catalyst component (c), is preferably not more than 20% by weight, more preferably not more than 10% by weight, based on 100% by weight of the cyclic ester compounds (a) and (b). As more detailed conditions for preparing the solid titanium catalyst component (I), the conditions described in, for example, EP585869A1 (European Patent Kokai No. 0585869) and the patent document 2 are preferably used, except that the cyclic ester compounds (a) and (b) are used. Olefin polymerization catalyst The olefin polymerization catalyst of the invention comprises: the above-mentioned solid titanium catalyst component (I) of the invention, and an organometallic compound catalyst component (II) containing a metallic element selected from the group 1, the group 2 and the group 13 of the periodic table. Organometallic compound catalyst component (II) As the organometallic compound catalyst component (II), a compound containing the group 13 metal, such as an organoaluminum compound, an alkylated complex compound of the group 1 metal and aluminum, or an oragnometallic compound of the group 2 metal is employable. Of such compounds, the organoaluminum compound is preferable. Preferred examples of the organometallic compound catalyst components (II) include organometallic compound catalyst components described in publicly known literatures such as the aforesaid EP585869A1. Electron donor (III) The olefin polymerization catalyst of the invention may contain the previously described electron donor (III) together with the organometallic compound catalyst component (II), when necessary. The electron donor (III) is preferably an organosilicon compound. The organosilicon compound is, for example, a compound represented by the following formula (3) . (Formula Removed) wherein R and R' are each a hydrocarbon group, and n is an integer of 0