A polyester resin that comprises an aromatic dicarboxylic acid as a principal dicarboxylic acid ingredient and an aliphatic diol as a principal diol ingredient (hereinafter this may be referred to as “aromatic polyester resin”), such as polyethylene terephthalate (PET) is characterized by having excellent mechanical properties, melt stability, solvent resistance, aroma preservability and recyclability.  Accordingly, the aromatic polyester resin is widely utilized for packaging materials such as films, sheets and hollow containers.  However, since the gas barrier properties thereof to oxygen, carbon dioxide and the like are not always good, the utilization thereof is limited in applications that require high gas barrier properties.  For imparting gas barrier properties to an aromatic polyester resin, there are known a method of laminating it with metal foil such as aluminum, a method of coating or laminating it with any other resin having high gas barrier properties, a method of coating it with aluminum or silicon by vapor deposition.  However, these methods are all problematic in that they may detract from transparency, they may require some complicated production step and they may detract from mechanical properties. 
For imparting high gas barrier properties to the resin without requiring any complicated production step, there is known a method of mixing the resin with some other resin having high gas barrier properties.  The resin having high gas barrier properties may be a polyamide resin such as typically nylon 6 and nylon 66; and in particular, a polyamide resin obtained through polymerization of metaxylylenediamine and adipic acid (hereinafter this may be referred to as “polyamide MXD6”) has excellent gas barrier properties.  On the other hand, there is known an ethylene/polyvinyl alcohol copolymer resin as another gas-barrier resin than polyamide resin.  The ethylene/polyvinyl alcohol copolymer resin is poorly compatible with aromatic polyester resin, and therefore, a composition comprising the two may be cloudy, and in addition, the copolymer resin has other problems in that its crystallinity is high and therefore it detracts from the stretchability of aromatic polyester resin, and its thermal stability is poor. 
On the other hand, polyamide MXD6 has high gas barrier properties, and its glass transition temperature, melting point and crystallinity are similar to those of aromatic polyester resin, especially polyethylene terephthalate, and further, the polyamide has excellent thermal stability in melting.  Accordingly, its advantages are that it may be readily melt-mixed with aromatic polyester resin, it does not detract from the mechanical properties and the stretchability of aromatic polyester resin, and it may express high gas barrier properties. 
However, a composition of an aromatic polyester resin and a polyamide resin such as polyamide MXD6 may give a pearly gloss depending on the dispersion condition and the concentration of the resin composition; and in particular, its glossiness may be more remarkable in thermal forming such as stretching, and its transparency may thereby lower.  To that effect, the transparency of the composition of an aromatic polyester resin and a polyamide resin such as polyamide MXD6 is insufficient, and therefore the use of the resin composition in applications requiring high transparency is limited. 
Patent Reference 1 proposes a composition prepared by adding a tetracarboxylic acid dianhydride to a mixture comprising a polyamide resin and a polyester resin.  Patent Reference 1 has a description relating to the improvement of the mechanical properties of shaped articles used as engineering plastics, but has no description at all relating to a resin composition for films, sheets and thin-wall hollow containers having improved transparency.  Patent Reference 2 proposes a compound having an epoxy group and an acid anhydride group as one type of a solubilizing agent for a composition of a thermoplastic polyester resin and a polyamide resin having a metaxylylene group in the main chain thereof.  However, the compound obviously differs from the compound in the present invention. 
Patent Reference 3 proposes a composition prepared by adding a tetracarboxylic acid dianhydride to a mixture comprising a polyamide resin and a polyester resin.  However, the tetracarboxylic acid dianhydride reacts with the polyester resin and the polyamide resin mixed with it.  Accordingly, the viscosity of the resin composition may excessively increase during mixing and shaping procedures, and the resin composition may be difficult to shape and work into films, sheets and thin-wall hollow containers.  In addition, the dianhydride added may color the composition in yellow to light brown, and has another problem in that it may detract from the appearance of the shaped articles. 
Patent Reference 4 proposes a composition prepared by adding a polycarboxylic acid having at least three carboxyl groups in one molecule and its anhydride to a mixture comprising a polyamide resin and a polyester resin.  However, for the same reasons as in the above, the polycarboxylic acid anhydride reacts with the polyester resin and the polyamide resin mixed with it; and therefore, the viscosity of the resin composition may excessively increase during mixing and shaping procedures, and the resin composition may be difficult to shape and work into films, sheets and thin-wall hollow containers.  In addition, the acid anhydride added may color the composition in yellow to light brown, and has another problem in that it may detract from the appearance of the shaped articles.  Accordingly, it is desired to develop a polyester resin composition having high gas barrier properties not requiring the complicated production steps as in the above, and having excellent transparency.     [Patent Reference 1] JP-A 1-272660    [Patent Reference 2] JP-A 62-201963    [Patent Reference 3] JP-A 2000-34357    [Patent Reference 4] JP-A 2000-302952