To provide a cipher key system inexpensive in facility cost which can effectively control the unlocking of a door without providing any information input terminal such as a card reader.

The reception timing of the major reflected component is obtained as the reflection characteristic of the ultrasonic wave in an area to be monitored from the reflected ultrasonic wave transmitted to the area A to be monitored (a reflection characteristic analysis unit 5). The series of the reflection characteristic successively detected by each timing when the position of an object in the area to be monitored is changed at the preset timing are registered in a memory 6 as a cipher key in implementing the predetermined process (a cipher key registering means), the reflection characteristic of the ultrasonic wave to be successively detected in monitoring the area to be monitored is compared with the series of the reflection characteristic of the cipher key registered in the memory in advance (a comparison and collation unit 7), and the implementation of the process such as the unlocking of the door is controlled according to the result of comparison.