An apparatus for evaluating gas barrier properties, comprising a support having a polymer for supporting a sample, a chamber on a permeation side, and a detection unit, wherein the support is joined to an opening of the chamber on a permeation side; a polymer film is provided between the support and a sample; a chamber on a supply side is provided such that an opening of the chamber on the supply side faces the opening of the chamber on the permeation side, the chamber on the supply side being disposed so as to be closely attachable to the sample and able to go up and down, such that the chamber on the supply side forms a tightly sealed space with the chamber on the permeation side, with the support, the polymer film and the sample being interposed therebetween, and the sample and the polymer film being fixed by being pressed to the support by the opening of the chamber on the supply side; an external chamber is provided on the permeation side and another external chamber is provided on the supply side; the space in the external chamber disposed on the outside of the upper external part of the chamber on the permeation side, and the chamber on the supply side, is covered by the external chamber on the permeation side and the external chamber on the supply side; an opening of the external chamber on the permeation side and an opening of the external chamber on the supply side face each other and are sealed; the external chamber on the supply side is able to go up and down; and an external chamber gas piping that leads to the space in the external chamber is connected to the external chamber on the permeation side; and the external chamber provided on the permeation side and the external chamber provided on the supply side cover a protective region interposed between the polymer film and the support which is formed naturally when the chamber on the supply side is pulled up and pressing from above is eliminated. a support having a polymer for supporting a sample, a chamber on a permeation side, and a detection unit, wherein the support is joined to an opening of the chamber on a permeation side; a polymer film is provided between the support and a sample; a chamber on a supply side is provided such that an opening of the chamber on the supply side faces the opening of the chamber on the permeation side, the chamber on the supply side being disposed so as to be closely attachable to the sample and able to go up and down, such that the chamber on the supply side forms a tightly sealed space with the chamber on the permeation side, with the support, the polymer film and the sample being interposed therebetween, and the sample and the polymer film being fixed by being pressed to the support by the opening of the chamber on the supply side; an external chamber is provided on the permeation side and another external chamber is provided on the supply side; the space in the external chamber disposed on the outside of the upper external part of the chamber on the permeation side, and the chamber on the supply side, is covered by the external chamber on the permeation side and the external chamber on the supply side; an opening of the external chamber on the permeation side and an opening of the external chamber on the supply side face each other and are sealed; the external chamber on the supply side is able to go up and down; and an external chamber gas piping that leads to the space in the external chamber is connected to the external chamber on the permeation side; and the external chamber provided on the permeation side and the external chamber provided on the supply side cover a protective region interposed between the polymer film and the support which is formed naturally when the chamber on the supply side is pulled up and pressing from above is eliminated.