1. An illegal communication detector that is mounted in a communication path between specific devices and a protection-targeted device to monitor contents of communications delivered to the protection-targeted device, the illegal communication detector comprising: a recorder that records at least part of information transmitted from the specific devices to the protection-targeted device; a detector that regards one of the specific devices as a device of interest and detects, when at least part of the recorded information transmitted from the device of interest complies with a predetermined illegal communication standard rule, that the contents of communication are derived from an illegal communication.