A microstrip resonator including a dielectric substrate, a resonance electrode
on a first main surface of the dielectric substrate, and a ground electrode over
an entire second main surface of the dielectric substrate. The resonance electrode
includes a superconducting film and a metal film deposited in that order. The ground
electrode includes a superconducting film and a metal film deposited in that order.
The superconducting film functions as an electrode in low-temperature operation
below a critical temperature, and the metal film functions as an electrode in high-temperature
operation at or above the critical temperature. The length of the superconducting
film of the resonance electrode is set to be longer than that of the metal film
of the resonance electrode, so that the resonance frequency in low-temperature
operation is substantially equal to the resonance frequency in high-temperature operation.
