Provided is (+)-5-(3,4-difluorophenyl)-5-[(3-methyl-2-oxopyridin-1(2H)-yl)m ethyl]imidazolidine-2,4-dione or a salt thereof. Also provided is a drug containing as the active ingredient (+)-5-(3,4-difluorophenyl)-5-[(3-methyl-2-oxopyridin-1(2H)-yl)m ethyl]imidazolidine-2,4-dione or a salt thereof.Se proporciona (+)-5-(3,4-difluorofenil)-5-[(3-metil-2-oxopiridin- 1(2H) -il)metil)imidazolidin-2,4-diona o una de sus sales. También se proporciona un fármaco que contiene como el ingrediente activo (+) -5- (3,4-difluorofenil) - 5-[(3-metil-2-oxopiridin-1(2H)-il)metil]iimidazolidin-2,4-diona o una de sus sales.