A boundary condition reflecting automatically, regardless of the direction of the gas flow, the pressure drop due to the entrance and exit effect occurred at a connection of two elements with different cross-sections in calculating the pressure profile of a vacuum network composed of many chambers and pipes which have different shape and vacuum properties was developed. The method of correcting the pressure drop in an element of varying cross-section like a cone was also introduced to keep the system free from directionality. The developed boundary conditions are applied as an example to a linear composed vacuum system to show how to make a set of simultaneous equations based on the particle balance equation and how to obtain its solution.