  We explored the parsec-scale nuclear regions of a sample of radio-loud
narrow-line Seyfert 1 galaxies (NLSy1s) using the VLBI Exploration of Radio
Astronomy (VERA) wideband (at a recording rate of $16\,\mathrm{Gbps}$)
polarimetry at 22 and 43 GHz. Our targets include 1H 0323+342, SBS 0846+513,
PMN J0948+0022, 1219+044, PKS 1502+036 and TXS 2116-077, which are all known to
exhibit $\gamma$-ray emission indicative of possessing highly beamed jets
similar to blazars. For the first time, we unambiguously detected Faraday
rotation toward the parsec-scale radio core of NLSy1s, with a median observed
core rotation measure (RM) of $2.7\times 10^3\,{\rm rad\,m^{-2}}$ (or
$6.3\times 10^3\,{\rm rad\,m^{-2}}$ for redshift-corrected). This level of RM
magnitude is significantly larger than those seen in the core of BL Lac objects
(BLOs; a dominant subclass of blazars), suggesting that the nuclear environment
of NLSy1s is more gas-rich than that in BLOs. Interestingly, the observed
parsec-scale polarimetric properties of NLSy1s (low core fractional
polarization, large core RM and jet-EVPA misalignment) are rather similar to
those of flat-spectrum radio quasars (FSRQs). Our results are in accordance
with the scenario that NLSy1s are in an early stage of AGN evolution with their
central black hole masses being smaller than those of more evolved FSRQs.
