PURPOSE: To obtain a coated optical fiber having improved transmission loss characteristics by heat-treating an optical quartz fiber coated with a resin composition contg. a catalyst for hydrogenation at a specified temp. in a prescribed atmosphere.

CONSTITUTION: A resin coated optical quartz fiber having one or more coating layers of a resin composition contg. about 0.01W10wt% catalyst for hydrogenation such as Ni is heat-treated at ≤200°C in an atmosphere of gaseous alkene or alkyne. The resin composition contains a compound having a carbon-carbon unsatd. bond such as 1-pentene besides urethane acrylate or the like as a principal component. A coated optical fiber generating very little hydrogen and hardly increasing the transmission loss with the lapse of time is obtd.