  Mathematical method of quantum phase space is very useful in physical
applications like quantum optics and non-relativistic quantum mechanics.
However, attempts to generalize it for the relativistic case lead to some
difficulties. One of problems is band structure of energy spectrum for a
relativistic particle. This corresponds to an internal degree of freedom,
so-called charge variable. In physical problems we often deal with such of
dynamical variables that do not depend on this degree of freedom. These are
position, momentum, and any combination of them. Restricting our consideration
to this kind of observables we propose the relativistic Weyl--Wigner--Moyal
formalism that contains some surprising differences from its non-relativistic
counterpart.
