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