For the multi-room ventilation calculations, bi- directional flows or counter flows in openings have been rarely taken into consideration and only uni-directional flows have been allowed for the calculation. It stands to reason that the calculation requires quite sophisticated scheme and the appearance of the bi-directional flows are restricted only to a limited number of openings neighboring the neutral plane of the building and also the flow rates may be too little to affect the total building ventilation. The existence of the bi-directional flows, however, is significant as it may promote the heat exchange between the adjacent rooms and affect the problem in the fire safety or the generation of the ventilation variety. In this paper, the modified ventilation calculation method using the network model is proposed, in which the effect of the bi-directional flows is partially included without the inordinate expansion of the computer capacity. The examination of the existence of the bi-directional flows, the errors in flow rates and the difference of the ventilation modes are investigated with the ordinary and modified ventilation calculati ns for the stairwell attached two- and three- story houses.