Microwave energy stored in a superconducting cavity is released to an output circuit in a short time by a gas discharge switch which is different from previous works. Output pulse of 350 times the input cw power fed to the cavity is obtained. A cw microwave is incident on a superconducting lead cavity of TE011cylindrical mode with resonant frequency 2.868 GHz and unloadedQof 3.8×106. A small semicircular glass tube with two electrodes and with rarefied helium gas is set inside the cavity. Pulsed discharge current of the order of 100 A and time width 0.4 &mgr;sec produces high‐density plasmas which may be equivalent to a metal loop antenna. Large microwave output pulses of time width 0.17 &mgr;sec are produced. It is suggested that the apparatus may be extended to millimeter microwave pulse production.