Program rewrite system for onboard electronic control device and onboard relay device

Provided area a program rewrite system for onboard electronic control devices (101), said program rewrite system being capable of reducing rewrite time when rewriting programs for multiprotocol-connected onboard electronic control devices (101), and an onboard relay device (200). In an embodiment, when a program for a multiprotocol-connected onboard electronic control device (101) is to be rewritten while in an onboard state, difference data between a new program and an old program are retrieved at a multiprotocol-compatible onboard relay device (200), and the difference data in the new program are transmitted to the onboard electronic control device (101) and stored therein.