A system for controlling a gas supply unit supplying gas to a tank includes a pressure acquisition unit configured to acquire the pressure in the tank; and a controller configured to perform first filling for supplying the gas to the tank and to perform second filling, based on a flow rate determined based on a change in the pressure in the tank for a predetermined period of time during the first filling, by controlling the gas supply unit. The controller is configured to cause the gas supply unit to perform the first filling at a first flow rate in a case where the pressure in the tank prior to the first filling is a first pressure and perform the first filling at a second flow rate exceeding the first flow rate in a case where the pressure is a second pressure exceeding the first pressure.