  We consider constructing the relativistic system of collective coordinates of
a field theory soliton on the basis of a simple principle: The collective
coordinates must be introduced into the static solution in such a way that the
equation of motion of the collective coordinates ensures that of the original
field theory. As an illustration, we apply this principle to the quantization
of spinning motion of the Skyrmion by incorporating the leading relativistic
correction to the rigid body approximation. We calculate the decay constant and
various static properties of nucleons, and find that the relativistic
corrections are in the range of 5% -- 20%. We also examine how the baryons
deform due to the spinning motion.
