An emission device includes an emission unit, and a controller. The emission unit emits a useful substance to a space where a target person is present. The controller controls the emission unit based on a circadian rhythm of the target person. The controller determines which one of a sympathetic nerve or a parasympathetic nerve is dominant in an autonomic nerve system of the target person, estimates the circadian rhythm of the target person in the autonomic nerve system from a result of the determination, determines a time to start emitting the useful substance based on the circadian rhythm estimated, and emits the useful substance from the emission unit when the time to start the emitting has come.