A superconducting device comprises a substrate, a non-superconducting layer formed in a principal surface of said substrate, an extremely thin superconducting channel formed of an oxide superconductor thin film on the non-superconducting layer. A superconducting source region and a superconducting drain region of a relatively thick thickness are formed of the oxide superconductor at the both sides of the superconducting channel separated from each other but electrically connected through the superconducting channel, so that a superconducting current can flow through the superconducting channel between the superconducting source region and the superconducting drain region. The superconducting device further includes a gate electrode through a gate insulator on the superconducting channel for controlling the superconducting current flowing through the superconducting channel, in which the superconducting channel is connected to the superconducting source region and the superconducting drain region at the higher portions than their one third height portions.