Percutaneous coronary intervention (PCI) is widely used as a treatment for ischemic heart disease. In particular, a high procedural precision of PCI for the left main coronary artery (LMCA), which supplies coronary artery blood to large amount of myocardium, is required. The restenosis after stent placement and target lesion revascularization of side branches (SB) is still a clinical issue. Regarding the stent implantation for bifurcation or trifurcation lesion, the stent struts should jail the inlets of SB. In such a situation, we clinically do the kissing balloon technique (KBT) to remove the struts existing at the opening of SB. As well as KBT, the proximal optimization technique (POT), which presses the stent against the wall, is also commonly used. However, merits of therapeutic strategies of KBT and POT for the branch are not confirmed from the fluid-dynamical viewpoint. In this study, we examined the influence of a stent in left main coronary artery on blood flow by employing CFD simulations. We investigated the influence of membrane position on the flow in bifurcation as well as of the stent position and shift on the flow in trifurcation. we confirmed that the effect of the stent position on the flow was bigger than that of the area occupied by the stent and that of KBT which moves the stent from the flow peak may be useful. However, it was also suggested that the intimal lining after KBT may cause serious flow inhibition.