Narrow-line Seyfert 1 galaxies (NLS1s) exhibit an 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 similar to 10(5-7)M(circle dot), and that a disk shines nearly at the Eddington luminosity, L-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 the mass-how rates, 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-bb similar or equal to 0.2(M/10(5)M(circle dot))(-1/4) keV, and the size of the blackbody emitting region is small, r(bb) less than or equal to 3r(S) (with r(S) being Schwarzschild radius), even for a Schwarzschild black hole. (3) All of the ASCA observation data of NLSls fall onto the region of M/(L-E/C-2) > 10 (with L-E being the Eddington luminosity) on the (r(bb):T-bb) plane, supporting our view that a slim disk emits soft X-rays at similar to L-E in NLS1s. (4) Magnetic energy can be amplified, at most, up to the equipartition value with the trapped radiation energy, which greatly exceeds the radiation energy emitted from the disk. Hence, energy release by consecutive magnetic reconnection will give rise to substantial variability in soft X-ray emission.