PURPOSE:To obtain the subject resin, excellent in affinity and adhesion of particles to a polyester and useful as magnetic tapes, etc., by adding particles having the surface treated with a specific aromatic compound to a system before completing polycondensing reaction. CONSTITUTION:A dicarboxylic acid containing an aromatic dicarboxylic acid as a principal acid component and an ester-forming derivative thereof and glycol are used to produce a polyester. In the process, particles having the surface treated with an aromatic compound expressed by the formula [R1 is sulfonic acid group or alkali metallic salt of the sulfonic acid group, carboxyl group or alkali metallic salt of the carboxyl group; R2 is sulfonic acid group or alkali metallic salt of the sulfonic acid group, carboxyl or alkali metallic salt of the carboxyl or H; R3 and R4 are COO(CH2)1OH, COOCmH2m+1 or CnH2n+1 ((l), (m) and (n) are 0-20)] are added to a system before completing the polycondensing reaction to afford the objective composition.