  The theorem known from Pauli equation about operators that anticommute with
Dirac's $K$-operator is generalized to the Dirac equation. By means of this
theorem the operator is constructed which governs the hidden symmetry in
relativistic Coulomb problem (Dirac equation). It is proved that this operator
coincides with the familiar Johnson-Lippmann one and is intimately connected to
the famous Laplace-Runge-Lenz (LRL) vector. Our derivation is very simple and
informative. It does not require a longtime and tedious calculations, as is
offten underlined in most papers.
