The present invention is a polyester composition for a masterbatch, which comprises a polyester, a magnesium compound, an alkali metal compound and a phosphorus compound, wherein magnesium atoms, alkali metal atoms and phosphorus atoms are contained in amounts of 400 to 2700 ppm by mass, 40 to 270 ppm by mass and 200 to 1700 ppm by mass, respectively, in the polyester composition, the melt resistivity of the polyester composition is 0.005×108 to 0.05×108 Ω·cm, and few contaminant particles are observed in the polyester composition under specified conditions. The polyester composition for a masterbatch has a satisfactorily low melt resistivity, contains few contaminant particles, and has an excellent color hue and excellent thermal stability.