  Based on the precision experimental data of energy-level differences in
hydrogenlike atoms, especially the 1S-2S transition of hydrogen and deuterium,
the necessity of establishing a reduced Dirac equation (RDE) with reduced mass
as the substitution of original electron mass is stressed. The theoretical
basis of RDE lies on two symmetries, the invariance under the space-time
inversion and that under the pure space inversion. Based on RDE and within the
framework of quantum electrodynamics in covariant form, the Lamb shift can be
evaluated (at one-loop level) as the radiative correction on a bound electron
staying in an off-mass-shell state--a new approach eliminating the infrared
divergence. Hence the whole calculation, though with limited accuracy, is
simplified, getting rid of all divergences and free of ambiguity.
