An increase in the amount of program code used in the firmware of electronic control units (ECUs) in vehicles has led to an increase in updates after sales to resolve bugs in the program code. In this situation, automakers are beginning to introduce over-the-air firmware update technology currently used in the mobile phone industry. We developed an incremental update method based on BSDiff and demonstrated its application to resource-constrained microcontrollers in ECUs. We implemented the method using a Renesas RH850/F1L simulator and evaluated the memory usage and compression ratio. We demonstrated that the proposed method is applicable to in-vehicle ECUs.