Abstract: An airtightness evaluation device includes a differential pressure gauge to output a pressure difference between two spaces connected to two joints. A system joint for connecting a refrigeration cycle system is connected to one joint of the differential pressure gauge. A pressure vessel is connected to the other joint of the differential pressure gauge. The airtightness evaluation device includes a bypass circuit to connect the system joint and the pressure vessel to each other with bypassing the differential pressure gauge, and a bypass valve to open/close the bypass circuit. The airtightness evaluation device also includes a supply source joint for supplying a pressurization gas to the pressure vessel.