To provide a permanent hair treatment agent for dyed hair capable of firmly waving hair treated with an oxidative hair-dyeing agent (colored hair) or straightening naturally curly hair and remarkably suppressing discoloration of the colored hair.

The permanent hair treating agent comprises a first agent comprising a reducing agent comprising thioglycolic acid, an alkali salt of thioglycolic acid, cysteine, an acid salt of cysteine or N-acetylcysteine and one or a combination of at least two selected from among polyols, sugar alcohols and sugars (except reducing monosaccharides) and a second agent comprising an oxidizing agent, where the polyols, the sugar alcohols and the sugars (except reducing monosaccharides) have a molecular weight within the range of 75-4,000, and the second agent is compounded in an amount within the range of 8-50 mass% based on the first agent.