  A relativistic spin operator is to be the difference between the total and
orbital angular momentum. As the unique position operator for a localized
state, the remarkable Newton-Wigner position operator, which has all desirable
commutation relations as a position operator, can give a proper spin operator.
Historically important three spin operators respectively proposed by Bogolubov
et al., Pryce, and Foldy-Woutheysen are investigated to manifest a
corresponding spin operator to the Newton-Wigner position operator. We clarify
a unique spin operator in relativistic quantum mechanics described by the Dirac
Hamiltonian.
