A gas filling method in which a gas filling system is used to fill a tank with a compressed gas from a supply source, the gas filling system comprising: the supply source in which the compressed gas is stored; a piping, connecting the supply source to the tank mounted on a movable body, wherein the piping is provided with a control valve, a pressure sensor, a gas temperature sensor and a flow rate sensor; and a control device, operating the control valve in a specific filling condition, and controlling a flow rate of the gas flowing through the piping; wherein the gas filling method comprises the following steps: an effective heat mass calculation step, which uses a heat capacity of the piping and a detection value of the gas temperature sensor to calculate a value of an effective heat mass (Heat mass) related to a temporary pressure loss caused by the piping before gas-filling is started; a pressure loss parameter calculation step, which uses a detection value of the pressure sensor when the flow rate of the gas in the piping changes to calculate a value of a pressure loss parameter related to a pressure loss caused by the piping after the gas-filling is started; and a filling condition changing step, which changes the filling condition into a condition that is defined based on the value of the pressure loss parameter and the gas-filling is continued. the supply source in which the compressed gas is stored; a piping, connecting the supply source to the tank mounted on a movable body, wherein the piping is provided with a control valve, a pressure sensor, a gas temperature sensor and a flow rate sensor; and a control device, operating the control valve in a specific filling condition, and controlling a flow rate of the gas flowing through the piping; wherein the gas filling method comprises the following steps: an effective heat mass calculation step, which uses a heat capacity of the piping and a detection value of the gas temperature sensor to calculate a value of an effective heat mass (Heat mass) related to a temporary pressure loss caused by the piping before gas-filling is started; a pressure loss parameter calculation step, which uses a detection value of the pressure sensor when the flow rate of the gas in the piping changes to calculate a value of a pressure loss parameter related to a pressure loss caused by the piping after the gas-filling is started; and a filling condition changing step, which changes the filling condition into a condition that is defined based on the value of the pressure loss parameter and the gas-filling is continued. an effective heat mass calculation step, which uses a heat capacity of the piping and a detection value of the gas temperature sensor to calculate a value of an effective heat mass (Heat mass) related to a temporary pressure loss caused by the piping before gas-filling is started; a pressure loss parameter calculation step, which uses a detection value of the pressure sensor when the flow rate of the gas in the piping changes to calculate a value of a pressure loss parameter related to a pressure loss caused by the piping after the gas-filling is started; and a filling condition changing step, which changes the filling condition into a condition that is defined based on the value of the pressure loss parameter and the gas-filling is continued.