An operation instrument includes an electric surgical knife, a liquid ejection surgical knife capable of ejecting liquid and an outer case configured to accommodate the electric surgical knife and the liquid ejection surgical knife, and including an opening which allows an electrode portion of the electric surgical knife to project therefrom. The liquid ejection surgical knife is operated so as to be switched between a therapy mode in which liquid is ejected to the outside of the outer case, and a cleaning mode configured to eject the liquid toward a high-frequency treatment electrode of the electric surgical knife in a state of being accommodated in the outer case.
