Data relay device for vehicle, and data relay method

A data relay method for a data relay device for a vehicle, the data relay device including a processor and a memory, the data relay method comprising:

• by the processor,
• communicating with an update device located outside the vehicle, the update device being configured to send an update execution signal;
• communicating with an internal device located inside the vehicle via an internal communication line;
• communicating with a specific device located inside the vehicle via a specific communication line different from the internal communication line,
• receiving update data for the internal device from the update device,
• receiving specific update data for the specific device from the update device;
• storing the update data and the specific update data in the memory,
• transmitting the update data stored in the memory via the internal communication line in response to receiving the update execution signal;
• transmitting the specific update data stored in the memory via the specific communication line in response to receiving the update execution signal;
• prohibiting transmitting message data from the internal device via the internal communication line in response to receiving the update execution signal;
• permitting transmitting specific message data from the specific device via the specific communication line in response to receiving the update execution signal;
• erasing the update data and the specific update data from the memory after transmitting the update data and the specific update data;
• and
• erasing the update data from the memory in a case where a data volume stored in the memory becomes equal to or larger than a first set volume and smaller than a second set volume before transmitting the update data and the specific update data, the first set volume being smaller than the second set volume.