  Narrow-line Seyfert 1 galaxies (NLS1s) exhibit extreme soft X-ray excess and
large variability. We argue that both features can be basically accounted for
by the slim disk model. We assume that a central black-hole mass in NLS1 is
relatively small, $M \sim 10^{5-7}M_\odot$, and that a disk shines nearly at
the Eddington luminosity, $L_{\rm E}$. Then, the disk becomes a slim disk and
exhibits the following distinctive signatures: (1) The disk luminosity
(particularly of X-rays) is insensitive to mass-flow rates, $\dot M$, since the
generated energy is partly carried away to the black hole by trapped photons in
accretion flow. (2) The spectra are multi-color blackbody. The maximum
blackbody temperature is $T_{\rm bb} \simeq 0.2(M/10^5 M_\odot)^{-1/4}$ keV,
and the size of the blackbody emitting region is small, $r_{\rm bb} \lsim 3
r_{\rm S}$ (with $r_{\rm S}$ being Schwarzschild radius) even for a
Schwarzschild black hole. (3) All the ASCA observation data of NLS1s fall onto
the region of $\dot M/(L_{\rm E}/c^2)>10$ (with $L_{\rm E}$ being the Eddington
luminosity) on the ($r_{\rm bb},T_{\rm bb}$) plane, supporting our view that a
slim disk emits soft X-rays at $\sim L_{\rm E}$ in NLS1s. (4) Magnetic energy
can be amplified, at most, up to the equipartition value with the trapped
radiation energy which greatly exceeds radiation energy emitted from the disk.
Hence, energy release by consecutive magnetic reconnection will give rise to
substantial variability in soft X-ray emission.
