<p>Disclosed is an orally administrable agent for prevention and/or treatment of multiple sclerosis, which is highly effective to multiple sclerosis and excellent in safety. Specifically disclosed is an agent for prevention and/or treatment of multiple sclerosis, which contains 2-benzyl-5-(4-chlorophenyl)-6-[4-(methylthio)phenyl]-2H- pyridazin-3-one or a solvate thereof as an active ingredient.</p>
