Provided is a gas filling method capable of accurately determining the initial pressure in a tank while minimizing increase in temperature and pressure regardless of the capacity of the tank. The gas filling method for connecting a supply source to a tank via a piping, and filling the tank with the gas, the method includes: an initial-filling initiation step of opening a flow control valve provided in the piping to start initial filling; a pressure-variation acquisition step of obtaining a pressure variation per unit time at a predetermined detection point downstream of the flow control valve in the piping; an initial-pressure detection step of obtaining a pressure at the detection point at a time specified using the pressure variation, and considering that pressure as the initial pressure in the tank; and a main-filling initiation step of starting main filling using the initial pressure.