We developed a new molecular dynamics (MD) scheme, introducing the concept of the potential buffering field through which an adsorbed phase could interact with an imaginary gas phase. This simulation cell allowed us to conduct a MD simulation that allowed a change in the number of molecules to attain equilibrium with given equilibrium pressure, like a grand canonical Monte Carlo simulation. By taking another choice for the setting of the cell, the number of molecules stayed constant but the equilibrium pressure was able to be obtained easily by a new technique of "particle counting method." The thus obtained equilibrium vapor phase pressure agreed with that obtained by Widom’s particle insertion method. Some adsorption simulations within slitlike pores of 2 and 3 nm were carried out Adsorption phenomena could be observed from monolayer adsorption on a pore wall under a low relative pressure Co the capillary condensation under a high relative pressure. Thus the adsorption equilibrium relation could be determined. The critical relative pressure for capillary condensation was smaller than that predicted by the modified Kelvin equation. This MD method shall provide much benefit in studying interfaces, which is important for analyzing condensation in pores.