The present technology is to provide a medical operation system, a surgical system, a surgical instrument, and an external force sensing system that detect a force acting on an end effector in a preferred manner. The medical operation system includes: an inner slave having an end effector； an outer slave into which the inner slave is inserted, the outer slave supporting the inner slave at a position that allows the end effector to protrude outward from an end of the outer slave； a strain detection unit that detects strain generated in the outer slave； and a processing unit that calculates a force acting on the end effector in a living subject, on the basis of a result of detection performed by the strain detection unit. The outer slave is a structure decoupled from the inner slave.
