  We report on optical photopolarimetric results of the radio-loud narrow line
Seyfert 1 (RL-NLSy1) galaxy PMN J0948+0022 on 2012 December to 2013 February
triggered by flux enhancements in near infrared and gamma-ray bands. Thanks to
one-shot polarimetry of the HOWPol installed to the Kanata telescope, we have
detected very rapid variability in the polarized-flux light curve on MJD 56281
(2012 December 20). The rise and decay times were about 140 sec and 180 sec,
respectively. The polarization degree (PD) reached 36 +/- 3% at the peak of the
short-duration pulse, while polarization angle (PA) remained almost constant.
In addition, temporal profiles of the total flux and PD showed highly variable
but well correlated behavior and discrete correlation function analysis
revealed that no significant time lag of more than 10 min was present. The high
PD and minute-scale variability in polarized flux provides a clear evidence of
synchrotron radiation from a very compact emission region of 10^14 cm size with
highly ordered magnetic field. Such micro variability of polarization are also
observed in several blazar jets, but its complex relation between total flux
and PD are explained by multi-zone model in several blazars. The implied single
emission region in PMN J0948+0022 might be reflecting a difference of jets
between RL-NLSy1s and blazars.
