A vehicle program rewriting system comprising: a first control unit configured to execute a first control program for controlling a vehicle; a second control unit provided separately from the first control unit and configured to execute a second control program for controlling the vehicle; and a first communication line (C1, C3, C5, C8) configured to connect, when rewriting of at least one of the first control program and the second control program is executed, the first control unit and a program update unit, the program update unit being configured to transmit program update information to the first control unit; characterized by a second communication line branching from the first communication line (C1, C3, C5, C8) and configured to connect the second control unit and the program update unit, wherein the second control unit is configured to, when the program update information for updating the second control program is transmitted from the program update unit, intercept communication content between the program update unit and the first control unit via the second communication line, determining whether update unit identification information identifying the program update unit is included in the communication content, when it is determined that the update unit identification information is included, determining whether second identification information identifying program update information for the second control program is included in the communication content, and when it is determined that the second identification information is included, acquiring the program update information for the second control program from the communication content. a first control unit configured to execute a first control program for controlling a vehicle; a second control unit provided separately from the first control unit and configured to execute a second control program for controlling the vehicle; and a first communication line (C1, C3, C5, C8) configured to connect, when rewriting of at least one of the first control program and the second control program is executed, the first control unit and a program update unit, the program update unit being configured to transmit program update information to the first control unit; characterized by a second communication line branching from the first communication line (C1, C3, C5, C8) and configured to connect the second control unit and the program update unit, wherein the second control unit is configured to, when the program update information for updating the second control program is transmitted from the program update unit, intercept communication content between the program update unit and the first control unit via the second communication line, determining whether update unit identification information identifying the program update unit is included in the communication content, when it is determined that the update unit identification information is included, determining whether second identification information identifying program update information for the second control program is included in the communication content, and when it is determined that the second identification information is included, acquiring the program update information for the second control program from the communication content.