An object of the invention is to provide a medical drug, a food and drink having a role in regulating the concentration of equal in vivo and being capable of taking for a long time with high safety and provide a selective medium for a microorganism having conversion ability to equal and a method of detecting the same. Provided are an equal concentration-raising or reducing agent containing a carbohydrate as an active ingredient; use of a carbohydrate for producing the equal concentration-raising or reducing agent; a method of raising or reducing the concentration of equal by administrating a carbohydrate in an effective dose; a selective medium containing a carbohydrate for a microorganism having conversion ability to equal; and a method of detecting a microorganism having conversion ability to equal by use of the selective medium.