Provided is a novel oxazolone derivative having an inhibitory activity on casein kinase 1d and casein kinase 1e. The inhibitor can inhibit casein kinase 1d and casein kinase 1e, and therefore can provide a medicinal agent useful for the treatment and/or prevention of diseases of which the disease condition are associated with the mechanism of activation of casein kinase 1d or casein kinase 1e. Particularly, the inhibitor can provide a medicinal agent useful for the treatment of circadian rhythm disorders (including sleep disorder), central nervous system degenerative diseases and cancer. An inhibitor of casein kinase 1d and casein kinase 1e, which comprises an oxazolone derivative represented by general formula (1), a salt of the derivative, or a solvate or hydrate of the derivative or the salt as an active ingredient. [In formula (1), X represents a halogen atom (which may be any one of a fluorine atom, a chlorine atom, a bromine atom and an iodine atom).]
