A method of giving an agent for improving a feed conversion rate for an egg-laying hen selected from all breeds of female chickens having a diet comprising the agent, the feed conversion rate being defined as a ratio of an amount of feed given to a total weight of eggs laid, the method comprising feeding the egg-laying hen a feed having acetylcysteine as an effective ingredient to improve the feed conversion rate, wherein the effective ingredient is fed to the egg-laying hen at a rate of about 1 mg/kg of body weight per day, and wherein the egg-laying hen is an adult hen capable of producing eggs.