We provide a novel model of cerebral circulation simulation for predicting the risk of hemodynamic infarction in severe stenosis or occlusion at the internal carotid artery (ICA) origin. This model used the patient-specific multiscale blood flow simulation, which was used to construct the three-dimensional (3D) model obtained from patient's data from time-of-flight magnetic resonance angiography, combined with the 1D and 0D models for peripheral branching arteries. In addition to the morphological information, the inflow and outflow boundary conditions were important for the patient-specific computational flow-dynamic 3D model with the arterial circle of Willis. Inflow volumes at the bilateral ICAs and vertebral arteries (VAs) were quantitatively measured by using phase-contrast magnetic resonance angiography (PC-MRA), whereas outflow volumes at the major intracranial arteries were measured by using iodoamphetamine single-photon emission computed tomography (IMP-SPECT). Each outflow volume (mL/min) on IMP-SPECT was calculated as follows: mean regional cerebral blood flow (mL·min^-1·100 g^-1 brain tissue) in each newly created vascular territory × voxel volume (voxel number × voxel size) × 1.04 (density of a human brain tissue) / 100, and revised by using the total inflow volume on PC-MRA. We evaluated 11 patients with ≥60% stenosis or occlusion at the unilateral ICA origin. The mean inflow volumes of the ICA and VA in the normal hemispheres were 296 and 153 mL/min, respectively. The mean outflow volumes of the anterior, middle, and posterior cerebral arteries (ACA, MCA, and PCA) in the normal hemispheres were 86,169, and 71 mL/min at rest, and 115,236, and 96 mL/min after the acetazolamide test, respectively. The increasing ratio of each outflow volume, termed cerebrovascular reserve (CVR), was ≥30% in the normal hemispheres. Three of 11 patients (case 5, 73% stenosis; case 8, 98% stenosis; case 11, occlusion) showed decreased CVR <10% of the ACA and/or the MCA at the side of the ICA severe stenosis. The stenosis ratio of the ICA origin was only one cause of the decreased outflow volume or CVR of the ACA or MCA; however, it was directly influenced by the inflow volume of the ICA. This novel multiscale blood flow simulation model is useful for predicting subsequent hemodynamic ischemic attacks.