An in-vehicle communication system includes: a plurality of local ECUs (vehicle control devices) connected to networks of buses, and configured to control an in-vehicle device; an OBDII port which is a communication port to an outside; and an illicit act detection sensor configured to detect an illicit act on a vehicle. Each local ECU includes a reprogramming receiver and a reprogramming controller. The reprogramming receiver receives a reprogramming request signal and reprogramming data from a reprogramming device connected to the OBDII port. Reprogramming for rewriting a software program of a target local ECU is executed according to the reprogramming request signal and the reprogramming data. When the illicit act detection sensor detects an illicit act, the reprogramming controller prohibits execution of reprogramming.