  In this paper the phase structure of the massive $\lambda \phi^4$ model at
finite temperature ($T \neq 0$) is investigated by applying a resummation
method inspired by the renormalization-group (RG) improvement to the one-loop
effective potential. The resummation method a la RG-improvement is shown to
work quite succesfully by resumming up systematically large correction-terms of
$O(\lambda T/\mu)$ and of $O(\lambda (T/\mu)^2)$. The temperature-dependent
phase transition of the model is shown to proceed through the second order
transition. The critical exponents are determined analytically and are compared
with those in other analyses.
