  We compare relativistic approximation methods, which describe gravitational
instability in the expanding universe, in a spherically symmetric model. Linear
perturbation theory, second-order perturbation theory, relativistic Zel'dovich
approximation, and relativistic post-Zel'dovich approximation are considered
and compared with the Lema\^{\i}tre-Tolman-Bondi solution in order to examine
the accuracy of these approximations. We consider some cases of inhomogeneous
matter distribution while the homogeneous top-hat model has been usually taken
in the previous Newtonian works. It is found that the Zel'dovich-type
approximations are generally more accurate than the conventional perturbation
theories in the weakly nonlinear regime. Applicable range of the
Zel'dovich-type approximations is also discussed.
