Rewriting system for vehicle controller

The rewriting device for rewriting data stored in a memory 16 of a vehicle controller 10 with new data is provided. The rewriting device 11 is capable of communicating with the vehicle controller. When deleting or writing operation on the memory is not being performed, the rewriting device determines that communication between the rewriting device and the vehicle controller is offline if no response from the vehicle controller within a first determination time. When deleting operation or writing operation on the memory is being performed, the determination of offline is prohibited until a second determination time has elapsed. The second determination time is greater than the first determination time. It is preferable that the second determination time for deleting operation is the time necessary to delete the data stored in the memory, and that the second determination time for writing operation is the time necessary to write the new data into the memory.