A fuel gas filling system including a plurality of storage tanks for storing fuel gas and a filling device configured to be selectively connected to the storage tanks and fill the fuel gas supplied from the storage tanks into a fuel tank of a vehicle, the filling system comprising: a fuel gas state detection unit (35, 212) configured to detect at least one of a pressure of the fuel gas supplied from the filling device and a temperature of the fuel gas in the fuel tank of the vehicle; a gas state fluctuation determination unit configured to determine whether or not there is a gas state fluctuation, the pressure or temperature of the fuel gas supplied from the filling device suddenly changing in the gas state fluctuation; and characterized by a filling control unit configured to execute each one of a first filling control and a second filling control, in the first filling control the fuel gas being filled into the vehicle from the filling device on the basis of at least one of the fuel gas pressure and the fuel gas temperature detected by the fuel gas state detection unit (35, 212), in the second filling control the fuel gas being filled into the vehicle according to the gas state fluctuation; wherein the filling control unit is configured to execute: the first filling control in normal time; and the second filling control of filling the fuel gas without using the fuel gas pressure detected by the fuel gas state detection unit (35, 212) when it is determined by the gas state fluctuation determination unit that there is the gas state fluctuation caused by the fuel gas pressure during fuel gas filling, or the second filling control of filling the fuel gas without using the fuel gas temperature detected by the fuel gas state detection unit (35, 212) when it is determined by the gas state fluctuation determination unit that there is the gas state fluctuation caused by the fuel gas temperature during fuel gas filling. a fuel gas state detection unit (35, 212) configured to detect at least one of a pressure of the fuel gas supplied from the filling device and a temperature of the fuel gas in the fuel tank of the vehicle; a gas state fluctuation determination unit configured to determine whether or not there is a gas state fluctuation, the pressure or temperature of the fuel gas supplied from the filling device suddenly changing in the gas state fluctuation; and characterized by a filling control unit configured to execute each one of a first filling control and a second filling control, in the first filling control the fuel gas being filled into the vehicle from the filling device on the basis of at least one of the fuel gas pressure and the fuel gas temperature detected by the fuel gas state detection unit (35, 212), in the second filling control the fuel gas being filled into the vehicle according to the gas state fluctuation; wherein the filling control unit is configured to execute: the first filling control in normal time; and the second filling control of filling the fuel gas without using the fuel gas pressure detected by the fuel gas state detection unit (35, 212) when it is determined by the gas state fluctuation determination unit that there is the gas state fluctuation caused by the fuel gas pressure during fuel gas filling, or the second filling control of filling the fuel gas without using the fuel gas temperature detected by the fuel gas state detection unit (35, 212) when it is determined by the gas state fluctuation determination unit that there is the gas state fluctuation caused by the fuel gas temperature during fuel gas filling. the first filling control in normal time; and the second filling control of filling the fuel gas without using the fuel gas pressure detected by the fuel gas state detection unit (35, 212) when it is determined by the gas state fluctuation determination unit that there is the gas state fluctuation caused by the fuel gas pressure during fuel gas filling, or the second filling control of filling the fuel gas without using the fuel gas temperature detected by the fuel gas state detection unit (35, 212) when it is determined by the gas state fluctuation determination unit that there is the gas state fluctuation caused by the fuel gas temperature during fuel gas filling.