The critical phenomena of optical bistability in two dissipative systems, i. e. the chemical reaction system and the Josephson junction, both driven externally by coherent light field, are examined by Landau (mean field) theory of phase transition in equilibrium thermodynamics. It is revealed that the critical point of bistability as an end point of firstorder-like transitions possesses the features of phase transition of second order. And the generality in the neibourhood of this critical point of these two systems with very different physical contents is illustrated also.