The KU-FEL (Kyoto University Free Electron Laser) facility uses an S-band 4,5 cell thermionic RF gun as an electron source. The gun is strongly suffering from back-bombarding effect. In order to mitigate the energy of back streaming electrons the used RF gun shall be modified to triode type. For this reason a RF driven pre-bunching small coaxial cavity should be attached to the main 4,5 cell gun. The coaxial cavity was designed and fabricated. We report here results of a hot test of the coaxial cavity.