To provide a communication system which is capable of informing and making a user pay attention if electric power of a mobile phone is consumed due to erroneous activation caused by so-called a noise.

When an electronic key 2 is activated due to reception of a signal different from a WAKE signal, information suggesting the electronic key is erroneously activated is stored in a memory 23a. A microcomputer 23 provides a security device 3 with the information, which is added to an ID code signal, when the microcomputer 23 performs both-way communication with the security device 3 if the information has been stored in the memory 23a. The security device 3 informs a user, using a display device 36, about the fact that electric power of the electronic key 2 is consumed due to erroneous activation caused by a noise when the security device 3 obtains the information from the electronic key 2. Therefore, a communication system can make the user pay attention to a safekeeping method of the electronic key 2, such as keeping the electronic key 2 under an environment where a noise does not influence on the electronic key 2.