A communication system constituted by a communication terminal apparatus and a wireless key apparatus which can carry out a wireless communication with the communication terminal apparatus by a predetermined wireless communication system, wherein said communication terminal apparatus comprises: a communication unit for carrying out a wireless communication by said predetermined wireless communication system, a distance judgment unit for judging a distance with respect to said wireless key apparatus, and a control unit for limiting a predetermined function of said communication terminal apparatus according to the distance with respect to said wireless key apparatus; said wireless key apparatus comprises: a communication unit for a wireless communication by said predetermined wireless communication system, and a distance judgment unit for judging a distance with respect to said communication terminal apparatus; at least one of said communication terminal apparatus and said wireless key apparatus comprises a transmission power changing unit for controlling the transmission power of a wireless signal which said communication unit of said one apparatus transmits; and when it is judged by said distance judgment unit that the distance with respect to said wireless key apparatus is longer than a first distance, said control unit of said communication terminal apparatus limits said predetermined function of said communication terminal apparatus and at the same time, said transmission power changing unit changes said transmission power such that the communication possible distance between said communication terminal apparatus and said wireless key apparatus becomes a second distance which is shorter than said first distance. a communication unit for carrying out a wireless communication by said predetermined wireless communication system, a distance judgment unit for judging a distance with respect to said wireless key apparatus, and a control unit for limiting a predetermined function of said communication terminal apparatus according to the distance with respect to said wireless key apparatus; said wireless key apparatus comprises: a communication unit for a wireless communication by said predetermined wireless communication system, and a distance judgment unit for judging a distance with respect to said communication terminal apparatus; at least one of said communication terminal apparatus and said wireless key apparatus comprises a transmission power changing unit for controlling the transmission power of a wireless signal which said communication unit of said one apparatus transmits; and when it is judged by said distance judgment unit that the distance with respect to said wireless key apparatus is longer than a first distance, said control unit of said communication terminal apparatus limits said predetermined function of said communication terminal apparatus and at the same time, said transmission power changing unit changes said transmission power such that the communication possible distance between said communication terminal apparatus and said wireless key apparatus becomes a second distance which is shorter than said first distance. a communication unit for a wireless communication by said predetermined wireless communication system, and a distance judgment unit for judging a distance with respect to said communication terminal apparatus; at least one of said communication terminal apparatus and said wireless key apparatus comprises a transmission power changing unit for controlling the transmission power of a wireless signal which said communication unit of said one apparatus transmits; and when it is judged by said distance judgment unit that the distance with respect to said wireless key apparatus is longer than a first distance, said control unit of said communication terminal apparatus limits said predetermined function of said communication terminal apparatus and at the same time, said transmission power changing unit changes said transmission power such that the communication possible distance between said communication terminal apparatus and said wireless key apparatus becomes a second distance which is shorter than said first distance.