To provide a method for preventing the oxidization of groundwater in a well in the nonuse of the well, and the well for use in the method.

The well 1 comprises: a column pipe 13 for collecting the groundwater inward; an injection section for injecting an inert gas into an upper section with respect to the groundwater in the column pipe 13; a cap 10 which is attached to an upper end of the column pipe 13 and which serves as a sealing member for sealing the inside of the column pipe 13 with the inert gas infilled in the column pipe 13; a concentration meter 9 for measuring the concentration of oxygen in the column pipe 13; and a detector 15 for detecting gas generated from under the ground and having the risk of an explosion etc. or doing harm to the human body.