  Recent VLBI (Very Long Baseline Interferometer) observations determined the
distances and proper motions of star-forming regions in spiral arms directly.
They showed that star-forming regions and young stars have large peculiar
motions, as large as 30 km/s with complex structures. Such a large peculiar
motion is incompatible with the prediction of the standard theory of
quasi-stationary spiral arms. We use a high-resolution, self-consistent
N-body+hydrodynamical simulation to explore how the spiral arms are formed and
maintained, and how star-forming regions move. We found that arms are not
quasi-stationary but transient and recurrent, as suggested in alternative
theories of spiral structures. Because of this transient nature of the spiral
arms, star-forming regions exhibit a trend of large and complex non-circular
motions, which is qualitatively consistent with the VLBI observations. Owing to
this large non-circular motion, a kinematically estimated gas map of our Galaxy
has a large systematic errors of ~2-3 kpc in the distance from the Sun.
