To enable rapid dark dyeing and to maintain dyeing power for a long time by dyeing a plastic lens in a dye liquid mixture added with chlorophenols as a dyeing assistant.

The plastic lens is dyed in the dye liquid mixture added with the chlorophenols as the dyeing assistant. The dye liquid mixture consists basically of the chlorophenols, the dyes and liquid media represented by water and is properly added with dye assistants of org. and inorg. systems, exclusive of the chlorophenols, and various kinds of additives, such as pH control agents, at need. The plastic lens to be dyed is a resin lens essentially consisting of an org. compd. The chlorophenols to be used are the compd. attached with chlor groups in the benzene ring of phenol and include, for example, o- chlorophenol, p-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, etc.