The object is to provide a lysis method, lysis treatment solution, detection method using an immunochromatographic device, and detection kit comprising an immunochromatographic device for detecting whether causative bacterium of mastitis is a staphylococcus or not by using milk of a livestock animal. There is provided a method for lysing a staphylococcus, which comprises the step of mixing a lysis agent containing a lytic enzyme, and at least one kind of ampholytic surfactant, and preferably further containing at least one kind of nonionic surfactant, with milk obtained form a livestock animal to lyse a staphylococcus existing in the milk. The lytic enzyme is preferably lysostaphin.