A serum resistin level reducing agent that comprises a cell wall disrupted product of a Chlorella pyrenoidos, and that reduces a serum resistin level, as an agent for oral administration. A food or beverage composition for reducing a serum resistin level that comprises a cell wall disrupted product of a Chlorella pyrenoidos, and that reduces a serum resistin level. A method of reducing a serum resistin level by applying a cell wall disrupted product of a chlorella for oral administration. A use of a cell wall disrupted product of a chlorella for oral administration for reducing a serum resistin level.