The first memory of a control unit has a control program stored therein, and an update of the control program is performed at a predetermined timing. After the update of the control program, the second memory in the control unit is referred to, and an event code stored in the second memory is extracted if the event code does not correspond to any one of event code group respectively associated with events that can possibly be generated by the updated control program. Due to the decreased necessity for use as reference data, the extracted event code may be considered as un-necessary and may be deleted, for example, from the memory.