To provide a portable terminal device, a communication system, and a control program capable of automatically changing function setting without user operation.

A portable terminal device 100 of this invention comprises a communication unit 101, a communication state detection unit 102, and a setting change unit 103. The communication unit 101 is capable of short-range radio communication with an opposite device 110 portable by a user. The communication state detection unit 102 detects the communication state of radio communication between the portable terminal device 100 and the opposite device 110. The setting change unit 103 changes the function setting of the portable terminal device 100 on the basis of a detection result obtained by the communication state detection unit 102.