3 a p superconductor layer 24 having the energy gap u03b4 2 and a p superconductor layer 25 having an energy gap u03b4 1 are successively piled on a substrate 1 which is made of an n superconductor having the energy gap u03b4 1 fig1 shows the energy gap distribution in the superconducting device as shown in fig1 the energy gap is made smallest at the superconductor layer 23 when a current is injected into the pn junction a quasi particle is excited only in the superconductor layer 23 and is excited to the energy level u03b4 3 since the layer 23 is sandwiched between the superconductor layers 22 and 24 having the energy gap u03b4 2 the excited quasi particle is relaxed at this time an electromagnetic wave having a wavelength corresponding to the difference between the energy gap u03b4 3 and the energy gap u03b4 2 is emitted in the superconducting device the total thickness of the superconductor layers 22 23 and 24 is made equal to an integral multiple of the wavelength of the electromagnetic wave in a medium where the electromagnetic wave is generated in this case the superconductor layers 22 23 and 24 can act as a resonator and an electromagnetic wave emitting path further an electromagnetic waveguide such as a fiber waveguide can be connected to the superconductor layers 22 23 and 24 when the length of the superconducting device is made equal to an integral multiple of the above wavelength of the electromagnetic wave the electromagnetic wave is repeatedly reflected from the end faces of the device thus the intensity of the electromagnetic wave is increased and a coherent electromagntic wave can be emitted from one of the.