PROBLEM TO BE SOLVED: To provide an agent, used for preventing or treating endometriosis, targeting midkine (MK) and to provide a method for examining endometriosis and to provide an examination medicine. SOLUTION: By studying the mitogen activity of MK on cultured endometrial stromal cells, it is clarified that MK stimulates the proliferation of ESC. By evaluating the MK concentration in the peritoneal fluid (PF), it is clarified that the MK level is elevated in a female with the progress of endometriosis. It is further found out that the MK concentration in the PF of a female under the therapy with GnRH agonist is significantly lower than the concentration in another group. It is indicated that MK mRNA is expressed in cells originating in peritoneal bone marrow, the peritoneal membrane and an intrauterine tissue. COPYRIGHT: (C)2008,JPO&INPIT