PURPOSE: To provide the titled optical glass made of phosphate glass contg. Cs2O and having a large Abbe's number, a positive large abnormal diffusion value and high transmittance in a shorter wavelength region and superior chemical durability.

CONSTITUTION: 0.1W8wt% Cs2O is added to a basic composition consisting of, by wt., 45W70% P2O5, 3W15% Al2O3, 0W10% B2O3, 0W10% Li2O, 0W25% Na2O, 0W30% K2O, 0W30% R2O (R is Li, Na and K), 0W10% MgO, 0W35% BaO, 0W20% CaO, 0W30% SrO and 0W40% R'O (R' is Mg, Ca, Sr and Ba) (15W45% R2O+R'O), and they are vitrified by melting at 1,200W1,300°C. Thus, phosphate optical glass having 1.49W1.58 refractive index and 71W66 Abbe's number is obtd.