To provide a method for filling a container with high pressure gas capable of filling the container with ultra high pressure gas at a low cost, and with necessary and enough quantity of gas having a reverse joule-Thomson effect, and also to provide a device used in the method.

After a seal plate (9) is placed on an opening part (12) of the container (11) sealed with high pressure gas with a gap (S) formed between the opening part (12) and the seal plate, the opening part is surrounded with an airtight space (4) together with the seal plate, the space and the internal part of the container are sealed with high pressure gas, the temperature and the pressure of which are regulated in a buffer container (3) communicated with the space through an on-off valve (6), by utilizing a pressure difference. The seal plate is contact-bonded to the opening part by a sealing means (5) moved forward and backward to the opening part in the space.