To provide a multiconductor flame-retardant coated optical fiber ribbon 5 having a good sectional shape of coated optical fibers and good flame retardancy, and free of an increase of optical transmission loss.

A plurality of optical fibers 1 are arranged and coated with an electron beam-curing resin blended with 15-50 wt.% of a powdery flame retardant such as magnesium hydroxide, and the resin is rapidly cross-linked with electron beams without exposure to air to obtain the objective flame-retardant coated optical fiber ribbon 5 having good shape characteristics and good flame retardancy also as the coated optical fiber ribbon 5, and free of an increase of optical transmission loss.