  We calculate multicolor light curves (LCs) of supernovae (SNe) from red
supergiants (RSGs) exploded within dense circumstellar medium (CSM). Multicolor
LCs are calculated by using a multi-group radiation hydrodynamics code STELLA.
If CSM is dense enough, the shock breakout signal is delayed and smeared by CSM
and kinetic energy of SN ejecta is efficiently converted to thermal energy
which is eventually released as radiation. We find that explosions of RSGs are
affected by CSM in early epochs when mass-loss rate just before the explosions
is higher than 10^{-4} Msun/yr. Their characteristic features are that the LC
has a luminous round peak followed by a flat LC, that multicolor LCs are
simultaneously bright in ultraviolet and optical at the peak, and that
photospheric velocity is very low at these epochs. We calculate LCs for various
CSM conditions and explosion properties, i.e., mass-loss rates, radii of CSM,
density slopes of CSM, explosion energies of SN ejecta, and SN progenitors
inside, to see their influence on LCs. We compare our model LCs to those of
ultraviolet-bright Type IIP SN 2009kf and show that the mass-loss rate of the
progenitor of SN 2009kf just before the explosion is likely to be higher than
10^{-4} Msun/yr. Combined with the fact that SN 2009kf is likely to be an
energetic explosion and has large 56Ni production, which implies that the
progenitor of SN 2009kf is a massive RSG, our results indicate that there could
be some mechanism to induce extensive mass loss in massive RSGs just before
their explosions.
