A control system for a quay crane a mounting platform position acquisition device, a land-side position acquisition device, a transportation vehicle position acquisition device, and a control device. The control system 30 is configured such that, when the control device predicts that a standby time will occur for a land-side cargo handling device based on a vehicle stop position, a current position of a relay mounting platform, a current position of the land-side cargo handling device, and a current position of an in-terminal transportation vehicle, the control device performs control that moves the relay mounting platform to a relay position which equalizes an actual land-side cycle time for the land-side cargo handling device including the standby time and a sea-side cycle time for a sea-side cargo handling device with each other.