PURPOSE: To apply excellent color deepening effects to a colored fiber structure by applying a resin containing a silicone-based resin and a polyethylene glycol derivative in combination thereto and subjecting the resultant structure to low- temperature plasma treatment.

CONSTITUTION: A resin containing a silicone-based resin, e.g., methylhydrogenpolysiloxane or dimethylpolysiloxane having amino or silanol groups, in an amount of 0.5-5% based on the fiber weight and a polyethylene glycol derivative in combination in an amount within the range of ≤0.5% based on the fiber weight is applied to a colored fiber structure (e.g., polyester fiber dyed to black) and the resultant structure is then subjected to low-temperature plasma treatment. For example, compounds expressed by the formula (R1 is alkyl or alkylaryl; R2 is H or methyl; m and n are 3-30) are cited as the polyethylene glycol derivative.