To obtain an agent for the treatment of a synthetic fiber for producing a carbon fiber capable of simultaneously and sufficiently preventing the welding of flame-retarded fibers in a flame-proofing treatment and the generation of the contaminating substance in a baking furnace in a carbonization process and provide a process for the treatment of the synthetic fiber for producing the carbon fiber.

A specific polyorganosiloxane derivative having a polyoxyalkylene modification group in the main chain is used as a lubricating agent to apply lubricity to a synthetic fiber for the production of a carbon fiber.