PURPOSE: To obtain the titled optical fiber having excellent appearance and heat resistance without foaming even in applying to a thick film by applying and baking a specific organic high polymer varnish onto the glass based optical fiber obtained by drawing an optical fiber base material or bundle thereof.

CONSTITUTION: A varnish of organic high polymer having imide group with ring closing ratio of ≥90% and/or varnish of organic high polymer having imide group and amide group with ring closing ratio of ≥90%, e.g., a varnish obtained by dissolving one or more kind of polymers having structures expressed by formulas I-V (R1 is group expressed by formula VI or formula VII; R2-3 are groups expressed by formula VIII and formula IX; R4 is group expressed by formula VI, formula VII or formula IX) in one or more kind of polar solvents such as N-methyl-2-pyrrolidone is applied and baked onto a glass based optical fiber obtained by drawing an optical fiber base material or bundle thereof.