A resin composition comprising a base resin consisting of a polyamide-based resin, an olefin-based resin, a polar group-modified polymer or a combination of two or more of them; wherein the base resin consists of a first thermoplastic resin other than a saponified ethylene-vinyl ester-based copolymer and saponified ethylene-vinyl ester-based copolymer post-modified by urethanation, acetalization, cyanoethylation, or oxyalkylenation; wherein the olefin-based resin is a lipophilic polymer in which main chain is constructed by only carbon-carbon bond, and is obtained by polymerizing olefin as a main monomer, which is an aliphatic hydrocarbon monomer having carbon-carbon double bond; wherein the polar group-modified polymer is a polymer which contains aliphatic hydrocarbon monomer units having carbon-carbon double bonds, and the polymer is modified with a polar group-containing compound; wherein the polar group-containing compound is a compound having a polar group such as a carboxyl group, an amino group, an alkoxyl group, a hydroxyl group, an amide group, an epoxy group, or an anhydride group; and a hydrate-forming alkaline earth metal salt satisfying water absorption properties (α) and (γ) shown below, wherein the hydrate-forming alkaline earth metal salt is completely dehydrated trimagnesium dicitrate having a water content falling in the range of 0 to 10 % by weight, the water content being determined through thermogravimetric analysis: (α) a ratio of (X 5) to, X 5 /Y ranging from 0.2 to 2.0, wherein X 5 is an amount of water absorption for 5 days per 100g of the alkaline earth metal salt when being placed for 5 days under a condition of 40 °C and 90 % relative humidity, and Y is a content of crystallization water in maximum hydrate of the alkaline earth metal salt; and (γ) a local maximum point exists in the amount of water absorption when the hydrate-forming alkaline earth metal salt is placed under the condition of 40 °C and 90 % relative humidity; wherein (X 5), and (γ) are determined according to the description. a base resin consisting of a polyamide-based resin, an olefin-based resin, a polar group-modified polymer or a combination of two or more of them; wherein the base resin consists of a first thermoplastic resin other than a saponified ethylene-vinyl ester-based copolymer and saponified ethylene-vinyl ester-based copolymer post-modified by urethanation, acetalization, cyanoethylation, or oxyalkylenation; wherein the olefin-based resin is a lipophilic polymer in which main chain is constructed by only carbon-carbon bond, and is obtained by polymerizing olefin as a main monomer, which is an aliphatic hydrocarbon monomer having carbon-carbon double bond; wherein the polar group-modified polymer is a polymer which contains aliphatic hydrocarbon monomer units having carbon-carbon double bonds, and the polymer is modified with a polar group-containing compound; wherein the polar group-containing compound is a compound having a polar group such as a carboxyl group, an amino group, an alkoxyl group, a hydroxyl group, an amide group, an epoxy group, or an anhydride group; and a hydrate-forming alkaline earth metal salt satisfying water absorption properties (α) and (γ) shown below, wherein the hydrate-forming alkaline earth metal salt is completely dehydrated trimagnesium dicitrate having a water content falling in the range of 0 to 10 % by weight, the water content being determined through thermogravimetric analysis: (α) a ratio of (X 5) to, X 5 /Y ranging from 0.2 to 2.0, wherein X 5 is an amount of water absorption for 5 days per 100g of the alkaline earth metal salt when being placed for 5 days under a condition of 40 °C and 90 % relative humidity, and Y is a content of crystallization water in maximum hydrate of the alkaline earth metal salt; and (γ) a local maximum point exists in the amount of water absorption when the hydrate-forming alkaline earth metal salt is placed under the condition of 40 °C and 90 % relative humidity; wherein (X 5), and (γ) are determined according to the description. (α) a ratio of (X 5) to, X 5 /Y ranging from 0.2 to 2.0, wherein X 5 is an amount of water absorption for 5 days per 100g of the alkaline earth metal salt when being placed for 5 days under a condition of 40 °C and 90 % relative humidity, and Y is a content of crystallization water in maximum hydrate of the alkaline earth metal salt; and (γ) a local maximum point exists in the amount of water absorption when the hydrate-forming alkaline earth metal salt is placed under the condition of 40 °C and 90 % relative humidity; wherein (X 5), and (γ) are determined according to the description.