If highly precise elucidation of the blood flow characteristics in a carotid bifurcation was possible, it would be widely applicable to diagnosis of circulatory diseases such as arteriosclerosis and cerebrovascular disease. This study was conducted to establish a new flow-dividing ratio estimation method applicable to an unsteady flow on a two-dimensional ultrasonic-measurement-integrated simulation of a carotid artery bifurcation for which it has been previously difficult to obtain a stable solution. In this new method, the flow-dividing ratio was directly adjusted by specifying the flow rate in a branch so that the difference of the Doppler velocities in the external carotid artery was decreased. The effectiveness of the proposed method was confirmed by a numerical experiment using the actual shape of a carotid artery bifurcation, and the superiority of the two-dimensional ultrasonic-measurement-integrated simulation over the ordinary simulation in terms of the reproducibility of the blood flow structure was clarified by analysis using clinical ultrasound data.