The present invention is to provide a medicinal composition for preventing or treating parasitic protozoan infections, having a high selective toxicity against parasitic protozoan infection, and a superior preventive or treating effect. A medicinal composition for preventing or treating parasitic protozoan infections comprises a compound shown by the following general formula (1) as an active ingredient (wherein R represents an alkyl group, aryl group or heterocyclic group; A and B each independently represents a 5- or 6-membered ring containing at least one hetero atom, or a condensation ring wherein 1 or more 3- to 8-membered ring is condensed thereto; Y represents S, O, Se, or —NR1— (R1 represents an alkyl group, aryl group or heterocyclic group); L1, L2, L3, L4 and L5 each independently represents a methine group; Q represents a physiologically acceptable anion; k represents an integer of 0 to 2, necessary to make the electric charge of the whole molecule 0; p and q each represents an integer of 0 to 3, wherein the sum of p and q is 1 or more, and 6 or less).