There is provided an inhibitor that 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 a disease, with the pathological condition of which the mechanism of activation of casein kinase 1δ or casein kinase 1ε is associated. Particularly, the above-described inhibitor is used to provide a pharmaceutical agent useful for the treatment of circadian rhythm disorder (including sleep disorder), central neurodegenerative disease, and cancer.An inhibitor of casein kinase 1δ and casein kinase 1ε, which comprises, 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, in the formula (1), each of R1 and R2 independently represents any one of a substituted or unsubstituted 6-membered or 5-membered heterocyclic group optionally having a condensed ring, a substituted or unsubstituted aromatic hydrocarbon group optionally having a condensed ring, and a substituted or unsubstituted aromatic hydrocarbon lower alkyl group or aromatic hydrocarbon lower alkenyl group optionally having a condensed ring.]