A liquid filling method of filling a liquid storage device including a first tank that stores a liquid to be supplied to an ejection head ejecting the liquid and a second tank that stores the liquid to be supplied to the first tank through a connection port provided in the first tank with the liquid, the method including: injecting the liquid from a liquid storage container into the first tank through a first liquid introduction port provided in the first tank, in a state where a second liquid introduction port provided in the second tank is closed.