A method of refilling liquid into a cartridge according to the present invention is a method of liquid refilling, through which liquid is refilled into a used cartridge (1) in a liquid jet apparatus, comprising: a film removing process, in which an injection hole film (90) adhered around an air discharge opening (21), which communicates with the interior of the used cartridge (1), to seal the air discharge opening (21) is removed in order to make the air discharge opening (21) open; a liquid injecting process, in which liquid is injected into the used cartridge (1) through the air discharge opening (21) that has been made open in the injection hole film piercing process; and an injection hole film rewelding process, in which the film is rewelded using a surface other than an originally welded surface as a rewelded surface in order to seal again the air discharge opening (21), through which the liquid is injected in the liquid injecting process.