There is provided a novel oxazolone derivative having inhibitory activity against casein kinase 1´ and casein kinase 1µ. In addition, the present inhibitor inhibits casein kinase 1´ and casein kinase 1µ, and thus there is also provided a pharmaceutical agent useful for the treatment and/or prevention of diseases, with the pathological conditions of which the activation mechanism of casein kinase 1´ or casein kinase 1µ is associated. There is further provided a pharmaceutical agent useful for the treatment of, particularly, circadian rhythm disorder (including sleep disorder), central neurodegenerative disease, and cancer. An inhibitor of casein kinase 1´ and casein kinase 1µ comprising, as an active ingredient, an oxazolone derivative represented by the following general formula (1), a salt thereof, a solvate thereof, or a hydrate thereof: wherein X represents a halogen atom (which may be any one of a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom).
