the present invention is completed as explained above if the super fet is manufactured in accordance with the first embodiment of the method of the present invention the limitation in the fine processing technique required for manufacturing the super fet is relaxed since the flatness of the upper surface of the superconducting device can be improved it become easy to form conductor wirings in a later process further more since the gate insulator is formed of an oxide which is made of the same composition and has same crystal structure as those of the oxide superconductor constituting the superconducting gate electrode and the superconducting channel the super fet is superior in mechanical stability and interfaces between the gate insulator and the superconducting gate electrode and between the gate insulator and the superconducting channel are formed sharp accordingly it is easy to manufacture the super fet with good repeatability and the manufactured super fet has a stable performance referring to fig2 a to 2h a second embodiment of the process for manufacturing the superconducting device will be described as shown in fig2 a there is prepared a substrate 5 similar to the substrate 5 shown in fig1 a as shown in fig2 b a c axis orientated y 1 ba 2 cu 3 o 7 u03b4 oxide superconductor thin film 1 having a thickness on the order of about 20 nanometers is deposited on the principal surface of the substrate 5.