The present invention provides a surveying instrument, which comprises rotating units operated rotatably, supporting units to support the rotating units, a reflection mirror portion provided on one of the supporting unit or the rotating unit, tilt sensors arranged on the other of the supporting unit and the rotating unit and for projecting a detecting light and for detecting a relative tilting of the reflection surface of the reflection mirror portion with respect to the supporting unit by receiving the detecting light reflected by the reflection mirror portion, and a control unit for calculating an unsteadiness of rotation of the rotating unit based on a signal from the tilt sensor.