  The two-dimensional hydrogen atom in an external magnetic field is considered
in the context of phase space. Using solution of the Schr\"{o}dinger equation
in phase space the Wigner function related to the Zeeman effect is calculated.
For this purpose, the Bohlin mapping is used to transform the Coulomb potential
into a harmonic oscillator problem. Then it is possible to solve the
Schr\"{o}dinger equation easier by using the perturbation theory. The
negativity parameter for this system is realised.
