A hydrogen filling system of the present invention is provided with: a pressure accumulator; a pipe connecting the pressure accumulator and a hydrogen tank on a vehicle; a flow volume control valve, a pressure sensor, and a flow volume sensor with which the pipe is fitted; and a station ECU which operates the control valve under a predetermined filling condition. A gas filling method for filling hydrogen gas into the tank from the accumulator is provided with: a step of, after filling of the hydrogen gas has been started, calculating, using a detected value from the pressure sensor when the flow volume of hydrogen gas in the pipe has decreased, the value of a pressure loss coefficient correlated with a pressure loss caused in the pipe; and a step of changing the filling condition to a condition determined on the basis of the value of the pressure loss coefficient.