To provide a binder for mats capable of readily obtaining a fiber mat having sufficient strength and hardly causing discoloring when thermally molded and the fiber mat using the binder.

The binder for mats comprises a crystalline unsaturated polyester obtained by polycondensing 0.5-20 mole% (based on alcohol component) at least 2-8C aliphatic dicarboxylic acid compound as an acid component with a monomer mixture of at least an alkylene (2-4C) oxide adduct (1-10 average added molar number) of bisphenol A with a 2-6C aliphatic diol as an alcohol component and having 0.6-1.3 peak temperature of softening point/heat of fusion and an aliphatic or alicyclic powdery free radical generator. The fiber mat is composed of the binder for mats and a fiber substrate.