An in-vehicle system comprising: a plurality of control devices including a master control device and a slave control device each of which is configured to: set, as a start storage area, any one of a plurality of storage areas set as storage areas that store software; and be started using the software stored in the start storage area, the master control device being configured to, when software stored in a storage area different from the start storage area is updated in the slave control device, transmit, to the slave control device, a switching instruction for setting the storage area that stores the updated software as the start storage area, the in-vehicle system being characterized in that: the slave control device is configured to implement, when receiving the switching instruction, setting of the start storage area at a time of a next start, and request, at every start, the master control device to execute a consistency determination processing for determining whether pieces of identification information of the software stored in the start storage area are consistent between the control devices, and the master control device is configured to, when receiving the request for execution of the consistency determination processing from the slave control device, execute the consistency determination processing. a plurality of control devices including a master control device and a slave control device each of which is configured to: set, as a start storage area, any one of a plurality of storage areas set as storage areas that store software; and be started using the software stored in the start storage area, set, as a start storage area, any one of a plurality of storage areas set as storage areas that store software; and be started using the software stored in the start storage area, the master control device being configured to, when software stored in a storage area different from the start storage area is updated in the slave control device, transmit, to the slave control device, a switching instruction for setting the storage area that stores the updated software as the start storage area, the in-vehicle system being characterized in that: the slave control device is configured to implement, when receiving the switching instruction, setting of the start storage area at a time of a next start, and request, at every start, the master control device to execute a consistency determination processing for determining whether pieces of identification information of the software stored in the start storage area are consistent between the control devices, and the master control device is configured to, when receiving the request for execution of the consistency determination processing from the slave control device, execute the consistency determination processing. the slave control device is configured to implement, when receiving the switching instruction, setting of the start storage area at a time of a next start, and request, at every start, the master control device to execute a consistency determination processing for determining whether pieces of identification information of the software stored in the start storage area are consistent between the control devices, and implement, when receiving the switching instruction, setting of the start storage area at a time of a next start, and request, at every start, the master control device to execute a consistency determination processing for determining whether pieces of identification information of the software stored in the start storage area are consistent between the control devices, and the master control device is configured to, when receiving the request for execution of the consistency determination processing from the slave control device, execute the consistency determination processing.