The object is to provide a lysis method and lysis treatment solution for efficiently lysing cells of various Streptococcus bacteria in milk of a livestock animal to release a specific antigen substance contained in the cells for detecting whether causative bacterium of mastitis is a Streptococcus bacterium or not by using the milk, as well as a detection method using an immunochromatographic device. There is provided a method for lysing a Streptococcus bacterium, which comprises the step of mixing a lysis agent containing a lytic enzyme with milk obtained form a livestock animal to lyse a Streptococcus bacterium existing in the milk. The lytic enzyme is preferably at least one selected from the group consisting of lysozyme, labiase, and β-N-acetylglucosaminidase.