This estimation device calculates a first perpendicular vector which is a component of a vector of movement of a surgical instrument to a first position which is a temporary pivot position, and is perpendicular to the axial direction of the surgical instrument, and a second vector which is a component of a vector of movement of the surgical instrument to a second position and is perpendicular to the axial direction. Using the inner product of the first and second perpendicular vectors, a first distance, which is the length from a reference point on the rear end side of the surgical instrument or a reference point on the front end side thereof to the first position, is updated.
