  Unstable state furnishes a semigroup irreducible representation of the
Poincar\'e group. The state vector is represented by a superposition of energy
eigenkets. As a consequence of this superposition, the state vector can be
transformed into the rest frame through {\it a} Lorentz transformation only
when the eigenkets are labeled by velocity variable, but not momentum variable.
We also clarify the meaning of the velocity variable in the state vector with
respect to the velocity derived from kinematical consideration of the
scattering process.
