The object of the present invention is to provide an early mastitis detection method that can detect mastitis easily and quickly when compared to the prior methods and also a biomarker to be used with the method.The object can be solved by a method of examining a mammary gland disease by using the level of cyclophilin A in a mammary gland or in milk as indicator. More specifically, the object can be a method of examining a mammary gland disease comprising steps (1) and (2) listed below; (1) a step of detecting cyclophilin A in the milk collected from an udder or an udder quarter of a subject and thereby determining the cyclophilin A level in the milk; and (2) a step of determining the onset of a mammary gland disease or the possibility of onset of a mammary gland disease in the udder or the udder quarter of the subject on the basis of the cyclophilin A level in the milk.