Electronic control units (ECUs) of in-vehicle systems are being designed to handle an increasing number of functions. However, malfunctions may occur when an in-vehicle system is upgraded. Moreover, because they are visible to hackers, connected vehicles are becoming the object of cyberattacks. Therefore, the necessity of updating ECU software is increasing. Car owners cannot use their cars during the update process. Therefore, reducing updating time is important. In this paper, we propose a method that reduces updating time by dividing the update data. During simulations, we found that the proposed method can slightly reduce updating time.