In-vehicle key check system having check history memory

An in-vehicle key check system has a security ECU, a transmitter and a receiver for communication with a portable electronic key. The ECU communicates with the electronic key each time a key check condition is satisfied, and checks whether the electronic key is an authorized one based on response signals from the electronic key. The check result indicating that the electronic key is the authorized one is stored in a memory. This check result is erased when a predetermined time elapses or when the electronic key is likely to be taken out from the vehicle. When another check condition is satisfied, the ECU refers to the memory and skips the key check operation if the previous check result is stored in the memory.