To provide a ground speed calculation method and a ground speed calculation device suitable for realizing inexpensive and highly-accurate detection of the ground speed by utilizing a ground speed detection device having a comparatively long positioning cycle and a sea speed detection device having a shorter positioning cycle than the ground speed detection device in a marine structure.

A sailing control system 1 has a constitution including an external environmental information input part 10 for inputting external environmental information such as a bow azimuth, the sea speed detection device 12 for detecting the speed of the marine structure against the water surface, the ground speed detection device 13 for detecting the speed of the marine structure against the ground surface, a driving force control device 14, and a propeller 15. The driving force control device 14 has a constitution including a driving force control part 140 including a target driving force calculation module 140a and an operation quantity calculation module, and a ground speed calculation part 141 for calculating an estimated value of the ground speed based on a detection result by the sea speed detection device 12 and a detection result by the ground speed detection device 13.