The present invention relates to a reagent kit for detecting biofilm in test tissue, the reagent kit comprising: (a) a pretreatment liquid comprising at least one surfactant selected, from the group consisting of nonionic surfactants, amphoteric surfactants, and cationic surfactants, (b) a staining liquid comprising a dye, and (c) a decolorizing liquid comprising at least one surfactant selected from the group consisting of nonionic surfactants, amphoteric surfactants, and cationic surfactants; wherein after a membrane is brought into contact with the test tissue and released from contact, the pretreatment liquid (a), the staining liquid (b), and the decolorizing liquid (c) are brought into contact with the contact surface of the membrane in this order.