  We have applied the recently proposed renormalization group improvement
procedure of the finite temperature effective potential, and have investigated
extensively the phase structure of the massive scalar $\phi^4$ model, showing
that the $\phi^4$ model has a rich 3-phase structure at $T \neq 0$, two of them
are not seen in the ordinary perturbative analysis. Temperature dependent phase
transition in this model is shown to be strongly the first order.
