Provided is a clock gene expression level regulator which has a long history of being eaten and is high in safety, easily obtainable and superior in processability. Specifically provided is a clock gene expression level regulator comprising, as an active component, at least one compound selected from the group consisting of oleuropein, 3,4-DHPEA-EA, derivatives thereof, and salts of the foregoing. The clock gene expression level regulator of the present invention can regulate the expression levels of the Per1 and Bmall genes in antiphase to each other, thereby entraining the expression rhythms of the Per and Bmal genes to their normal phases.