Bubble nucleation phenomenon in presence of gas impurities is studied by microcanonical molecular dynamics simulation for Lennard-Jones fluid. First, we investigate effect of molecular diameter of dissolved gas on the nucleation rate with pressure change. As a result, we find increase of the nucleation rate with increase of the pressure, which we call “inverse” pressure phenomenon, when interaction of dissolved gas is very weak. In this case, we confirm that a gas with the larger diameter enhances composition fluctuation or phase separation. Next, we estimate effect of the diameter on spinodal point through calculation of equation of state (EOS), and confirm that effect of the diameter on spinodal point at low temperature range becomes larger as the interaction of dissolved gas becomes weaker. Finally, we examine applicability of the superheat ratio expressed by saturation pressure and spinodal pressure, and confirm its validity to explain the inverse change of the nucleation rate.