PROBLEM TO BE SOLVED: To provide a cheap fluorescent dye using biomass as a raw material being cheap and constantly available in a large mass, the dye fluorescing stably for a long period of time without flickering.SOLUTION: Lignin extracted from biomass by using an aqueous solution of acetic acid is used as the main component of the fluorescent dye of the invention. If desired, a mineral acid such as sulfuric acid may be used with the acetic acid aqueous solution for extraction. Since the main component lignin is a stable material, irradiation of excitation light does not cause color fading, and fluorescent light does not flicker. Further, the lignin can be stored at room temperature making its management easy. Also, because of its stability, the fluorescent dye may be micronized to enable its introduction into living cells. The fluorescent dye of the invention strongly fluoresces when irradiated with an excitation light of a wavelength around from 488 nm to 566 nm, so that it is useful as the fluorescent dy
