  We report the detection of a very short time scale flare in the radio-loud
narrow line quasars PKS 0558-504 by using ASCA. The X-ray count rates increased
and decreased by a factor of two in 33 minutes, and possibly by 40% in as short
as two minutes during this flare, confirming that such flare event does occur
in this object with imaging detectors. The implied largest rate of change in
luminosity in 0.8-10 keV alone, dL/dt=(1.8+/-0.4) 10^{42} ergs s^-2, is several
times higher than the limit sets for the isotropic emitting plasma around a
Kerr black hole. Either emission from a relativistic boosting jet or a magnetic
heated corona may explain such high radiative efficiency. Magnetic field with a
strength of at least a few 10^4 Gauss is required in the latter case. The
spectrum during this flare is significant harder than the average one. Three
radio loud narrow line active galactic nuclei (AGN) possess considerably
smaller black holes than in radio loud AGN in bright quasar survey, indicating
they are still in a phase of the rapid growth of the black hole by accretion.
