  ASCA observations of Narrow-Line Seyfert 1 galaxies (NLS1s) are presented. We
focus on the black hole size of the NLS1 sources by employing two independent
methods for the mass estimation; one is using X-ray variability, the other is
using a blackbody fit to the soft component. Although the coincidence is not
good for some sources, the mass estimated by these methods ranges from 1e5 to
1e7 solar masses, systematically smaller than those for typical (broad line)
Seyfert 1. We consider the small mass black hole to be the principal cause of
the several extreme characteristics of the NLS1s.
