  Planetary migration is one of the most serious problems to systematically
understand the observations of exoplanets. We clarify that the theoretically
predicted type II migration is too fast, as well as type I migration, by
developing detailed analytical arguments in which the timescale of type II
migration is compared with the disk lifetime. In the disk-dominated regime, the
type II migration timescale is characterized by a {\it local} viscous diffusion
timescale, while the disk lifetime characterized by a {\it global} diffusion
timescale that is much longer than the local one. Even in the planet-dominated
regime where the inertia of the planet mass reduces the migration speed, the
timescale is still shorter than the disk lifetime except in the final disk
evolution stage where the total disk mass decays below the planet mass. This
suggests that most giant planets plunge into the central stars within the disk
lifetime, and it contradicts the exoplanet observations that gas giants are
piled up at $r \ga 1$AU. We examine additional processes that may arise in
protoplanetary disks: dead zones, photoevaporation of gas, and gas flow across
a gap formed by a type II migrator. Although they make the type II migration
timescale closer to the disk lifetime, we show that none of them can act as an
effective barrier for rapid type II migration with the current knowledge of
these processes. We point out that gas flow across a gap and the fraction of
the flow accreted onto the planets are uncertain and they may have a potential
to solve the problem. Much more detailed investigation for each process may be
needed to explain the observed distribution of gas giants in extrasolar
planetary systems.
