An in-vehicle relay device includes: a first determination unit that determines whether or not first data is included in a received communication signal; a non-volatile storage unit that sequentially stores the first data that has been determined by the first determination unit as being included; a second determination unit that determines whether or not second data is included in the received communication signal; and a prohibition unit that, when the second determination unit has determined that the second data is included, prohibits the storage to be performed by the storage unit.