  A high Q-factor microwave resonator in a high magnetic field could be used in
a wide range of applications, especially for enhancing the scanning speed in
axion dark matter research. In this letter, we introduce a polygon-shaped
resonant cavity with commercial YBCO tapes covering the entire inner wall. We
demonstrated that the maximum Q-factor (TM$_{010}$, 6.93 GHz) of the
superconducting YBCO cavity was about 6 times higher than that of a copper
cavity and showed no significant degradation up to 8 T at 4 K. This is the
first indication of the possible applications of HTS technology to the research
areas requiring low loss in a strong magnetic field at high radio frequencies.
