A ground control point device 100 is set up at a control point of a Synthetic Aperture Radar (SAR) geodetic system in which SAR analysis. The ground control point device 100 comprises an SAR microwave reflector 10 configured to reflect an SAR microwave incident from an SAR toward an incident direction, a Global Navigation Satellite System (GNSS) receiver 20 configured to receive a GNSS wave to generate, based on the GNSS wave, positional information, which indicates a position of the control point, a ground state detector 30 configured to detect a state of a ground under the control point, the state including at least a moisture content rate of the ground, and a control point data transmitter 40 configured to transmit, to outside, control point data, which includes the positional information and a detection value of the ground state detector 30.
