A binder composition for making foundry molds, comprising an acid-hardening resin, one or more 5-position-substituted-furfural compounds selected from the group consisting of 5-hydroxymethylfurfural and 5-acetoxymethylfurfural, and one or more compounds selected from the group consisting of urea and derivatives of urea, wherein the content of the 5-position-substituted-furfural compound(s) by percentage is from 1 to 60% by weight of the binder composition. The content of the compound(s) by percentage selected from the group consisting of urea and derivatives of urea is preferably from 0.3 to 10% by weight of the binder composition.