Effective Potential at Finite Temperature

We have applied the recently proposed renormalization group improvementprocedure of the finite temperature effective potential, and have investigatedextensively the phase structure of the massive scalar $\phi^4$ model, showingthat the $\phi^4$ model has a rich 3-phase structure at $T \neq 0$, two of themare not seen in the ordinary perturbative analysis. Temperature dependent phasetransition in this model is shown to be strongly the first order.Comment: 3 pages, latex, 3 figures (not include, but send by request). Talk given at YITP International Workshop on Physics of Relativistic Heavy Ion Collisions, Kyoto, Japan, 9-11 June 1997, to be published in Prog. Theor. Phys. Supplement (Proceeding volume