A prophylactic/therapeutic agent for adult T cell leukemia comprising a MET receptor inhibitor such as NK4, DNA encoding NK4 or anti-HGF antibody. The agent can inhibit the activation of the HGF-MET system (which is a disease progress mechanism for adult T cell leukemia) and is therefore useful for the prevention/treatment of adult T cell leukemia which is a leukemia induced by the infection of T cells with human T cell leukemia virus type I.