The present invention relates to a surveying system, comprising a rotary laser irradiation device 1 installed at a known point and used for projecting two or more fan-shaped laser beams 3 with at least one of the fan-shaped laser beams tilted and for forming a laser reference plane, a photodetection device 4, which is mounted on a mobile support member 9, for receiving the laser beam and for calculating an elevation angle at a photodetecting position, and a monitor photodetection device 11 installed at a known position within the laser reference plane, wherein the monitor photodetection device has a monitor photodetection sensor for sensing and detecting the laser beam, a storage unit for storing a reference value of the laser reference plane, an arithmetic control unit for calculating positional changes over time of the laser reference plane based on the result of photodetection by the monitor photodetection sensor and on the reference value, and a transmitting unit for transmitting the positional changes over time to the photodetection device and/or to the rotary laser irradiation device as compensation information.