The semiclassical quantum theory is investigated with conformal mappin
g to analyze the energy levels for a central force potential in two di
mensions. The conformal mapping produces a modification in the effecti
ve potential which provides the correct phase for the WKB wave functio
n. With this modification in the effective potential the semiclassical
quantum theory, the so-called WKB approximation, yields the exact ene
rgy spectrum for a central force potential in two dimensions, such as
the parabolic potential, the attractive Coulomb potential, and the don
or impurity states under a plane magnetic field in a two-dimensional s
ystem. Among these, the result for the last one is compared with exper
imental results and other numerical results.