There is provided a novel oxazolone derivative having inhibitory activity against casein kinase 1δ and casein kinase 1ε. In addition, the present inhibitor inhibits casein kinase 1δ and casein kinase 1ε, 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 1δ or casein kinase 1ε 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 1δ and casein kinase 1ε comprising, as an act 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).
