Piercing device sealed container using the measuring method and to the amount of oxygen sealed container

An object is to offer a method that can efficiently measure the oxygen content of both the headspace and fluid within a hermetically sealed container by taking advantage of the characteristics of a fluorescent-type oxygen concentration meter. A container 2 is pierced using a hollow needle 28 with the interior space 28a sealed from the outside and the tip of said hollow needle 28 removed, and the interior space 28a is made continuous with the headspace within the container 2; a probe 11 of a fluorescent oxygen concentration meter 10 is inserted through the hollow needle 28 to the interior of the container 2; the fluorescent material of the probe 11 is positioned in the headspace 3 within the container 2, and the oxygen concentration of the headspace 3 is measured; and the probe 11 is inserted further so that the fluorescent material is submerged in the fluid 4 inside the container 2, and the dissolved oxygen concentration inside the fluid 4 is measured.