Provided is a medicinal composition, in particular, a medicinal composition for treatment and/or prevention which has a high therapeutic effect on infection with a parasitic protozoa and a selective toxicity thereto and exhibits a life-prolonging effect and so on. A medicinal composition which contains as the active ingredient a benzo[a]phenoxazine compound represented by General formula (1) or a salt compound of the same, in particular, an agent for treating and/or preventing infection with a protozoa such as malaria, leishmaniasis, African trypanosomiasis, Chagas disease, toxoplasmosis, lymphatic filariasis, babesiosis or coccidium disease; and a novel compound which is contained therein as the active ingredient.