Abstract: An optical plummet mechanism for compensating an error due to an inclination of a surveying instrument comprises a first pendulum suspended in the instrument body interior, a second pendulum suspended in a perpendicular relation to the first pendulum, a first reflector provided on the first pendulum for reflecting light incident in the direction of a collimation axis of a collimation lens in the direction of a substantially horizontal collimation axis lying in the plane of swinging of the first pendulum, and a second reflector provided on the second pendulum and being a double reflecting surface reflector for reflecting light incident in the direction of a collimation axis lying the the plane of swinging of the first pendulum in the direction of a vertical collimation axis. Collimation errors due to inclination of the instrument body in two directions, i.e.