Tilt detecting device and surveying instrument

A tilt detecting unit has a tilt sensor and is rotatably supported by an inner frame through a second shaft orthogonal to a first shaft, a first encoder is provided on the first shaft, a second encoder is provided on the second shaft, motors are provided on the respective shafts, an arithmetic processing module drives the respective motors so that the tilt sensor detects the horizontality based on a signal issued by the tilt sensor, calculates a tilt of the outer frame based on the outputs from the first and second encoders when the tilt sensor detects the horizontality, reverses the tilt detecting unit 180° at least once based on the outputs from the respective encoders in a stationary state of the tilt detecting device, and performs the calibration of the tilt sensor based on the detection signals output from the tilt sensor before and after the reversal.