The purpose of the present invention is to provide a system such that functions of a vehicle control system can be quickly reconfigured. The present invention is a processing device connected to at least one processing device, wherein during a period in which the one processing device executes a control operation on the basis of a control program installed in the one processing device, the processing device acquires a substitute program for the control program. In another aspect, the present invention is an on-board control system equipped with multiple processing devices, wherein during a period in which one processing device among the multiple processing devices executes a control operation on the basis of a control program installed in the one processing device, another processing device acquires a substitute program for the program in the one processing device.