  Shock breakout is the brightest radiative phenomenon in a Type II supernova
(SN). Although it was predicted to be bright, the direct observation is
difficult due to the short duration and X-ray/ultraviolet-peaked spectra. First
entire observations of the shock breakouts of Type II Plateau SNe (SNe IIP)
were reported in 2008 by ultraviolet and optical observations by the {\it
GALEX} satellite and supernova legacy survey (SNLS), named SNLS-04D2dc and
SNLS-06D1jd. We present multicolor light curves of a SN IIP, including the
shock breakout and plateau, calculated with a multigroup radiation
hydrodynamical code {\sc STELLA} and an evolutionary progenitor model. The
synthetic multicolor light curves reproduce well the observations of
SNLS-04D2dc. This is the first study to reproduce the ultraviolet light curve
of the shock breakout and the optical light curve of the plateau consistently.
We conclude that SNLS-04D2dc is the explosion with a canonical explosion energy
$1.2\times10^{51}$ ergs and that its progenitor is a star with a zero-age
main-sequence mass $20M_\odot$ and a presupernova radius $800R_\odot$. The
model demonstrates that the peak apparent $B$-band magnitude of the shock
breakout would be $m_{\rm B}\sim26.4$ mag if a SN being identical to
SNLS-04D2dc occurs at a redshift $z=1$, which can be reached by 8m-class
telescopes. The result evidences that the shock breakout has a great potential
to detect SNe IIP at $z\gsim1$.
