  We report the finding of kiloparsec (kpc)-scale radio structures in three
radio-loud narrow-line Seyfert 1 (NLS1) galaxies from the Faint Images of the
Radio Sky at Twenty-centimeters (FIRST) of the Very Large Array (VLA), which
increases the number of known radio-loud NLS1s with kpc-scale structures to
six, including two gamma-ray emitting NLS1s (PMN J0948+0022 and 1H 0323+342)
detected by the Fermi Gamma-ray Space Telescope. The detection rate of extended
radio emissions in NLS1s is lower than that in broad-line active galactic
nuclei (AGNs) with a statistical significance. We found both core-dominated
(blazar-like) and lobe-dominated (radio-galaxy-like) radio structures in these
six NLS1s, which can be understood in the framework of the unified scheme of
radio-loud AGNs that considers radio galaxies as non-beamed parent populations
of blazars. Five of the six NLS1s have (i) extended radio luminosities
suggesting jet kinetic powers of >~10^44 erg/s, which is sufficient to make
jets escape from hosts' dense environments, (ii) black holes of >~10^7 solar
mass, which can generate the necessary jet powers from near-Eddington mass
accretion, and (iii) two-sided radio structures at kpc scales, requiring
expansion rates of ~0.01c--0.3c and kinematic ages of >~10^7 years. On the
other hand, most typical NLS1s would be driven by black holes of <~10^7 solar
mass in a limited lifetime of ~10^7 years. Hence the kpc-scale radio structures
may originate in a small window of opportunity during the final stage of the
NLS1 phase just before growing into broad-line AGNs.
