An antiparasitic agent for fish includes an inhibitor of folate synthesis and/or an inhibitor of folate activation as the active substance(s). A combination preparation composed of an inhibitor of folate synthesis and an inhibitor of folate activation is disclosed. Sulfonamide is disclosed as suitable for the inhibitor of folate synthesis. A dihydrofolate reductase inhibitor, a folate antagonist are disclosed as suitable inhibitors of folate activation. The antiparasitic agent is able to exterminate fish parasites via oral administration. It is effective against fish parasites belonging to the ciliate group.