A testing device includes a pressure vessel, a mounting stand disposed in an internal space of the pressure vessel, on which a device to be tested is mounted, test electrodes, disposed in the internal space of the pressure vessel, that supply a test voltage to the device to be tested mounted on the mounting stand, and a pressurization unit that raises the pressure of the internal space of the pressure vessel. The test voltage is supplied from the test electrodes to the device to be tested mounted on the mounting stand, and testing of the device to be tested is carried out, in a condition that the pressure of the internal space of the pressure vessel is raised by the pressurization unit.