A portable electronic device comprising: a radiator; a communication circuit electrically connected to the radiator, the communication circuit being configured to transmit or receive a signal using the radiator; a sensor circuit electrically connected to the radiator and configured to measure a capacitance corresponding to an external object adjacent to the radiator; a frequency adjustment circuit electrically connected to the radiator, the frequency adjustment circuit being configured to be capable of adjusting a resonant frequency of the radiator; and a processor, wherein the processor is configured to: determine the capacitance corresponding to the external object using the sensor circuit while communication is being performed with an external device using the communication circuit; determine whether or not the portable electronic device is held by a user based on the determined capacitance; if it is determined that the portable electronic device is held by the user: designate an operation mode of the frequency adjustment circuit as a first mode when the capacitance falls within a first designated range, corresponding to a left-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the first mode; and designate the operation mode of the frequency adjustment circuit as a second mode when the capacitance falls within a second designated range, corresponding to a right-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the second mode. a radiator; a communication circuit electrically connected to the radiator, the communication circuit being configured to transmit or receive a signal using the radiator; a sensor circuit electrically connected to the radiator and configured to measure a capacitance corresponding to an external object adjacent to the radiator; a frequency adjustment circuit electrically connected to the radiator, the frequency adjustment circuit being configured to be capable of adjusting a resonant frequency of the radiator; and a processor, wherein the processor is configured to: determine the capacitance corresponding to the external object using the sensor circuit while communication is being performed with an external device using the communication circuit; determine whether or not the portable electronic device is held by a user based on the determined capacitance; if it is determined that the portable electronic device is held by the user: designate an operation mode of the frequency adjustment circuit as a first mode when the capacitance falls within a first designated range, corresponding to a left-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the first mode; and designate the operation mode of the frequency adjustment circuit as a second mode when the capacitance falls within a second designated range, corresponding to a right-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the second mode. determine the capacitance corresponding to the external object using the sensor circuit while communication is being performed with an external device using the communication circuit; determine whether or not the portable electronic device is held by a user based on the determined capacitance; if it is determined that the portable electronic device is held by the user: designate an operation mode of the frequency adjustment circuit as a first mode when the capacitance falls within a first designated range, corresponding to a left-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the first mode; and designate the operation mode of the frequency adjustment circuit as a second mode when the capacitance falls within a second designated range, corresponding to a right-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the second mode. designate an operation mode of the frequency adjustment circuit as a first mode when the capacitance falls within a first designated range, corresponding to a left-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the first mode; and designate the operation mode of the frequency adjustment circuit as a second mode when the capacitance falls within a second designated range, corresponding to a right-hand grip, and perform the communication using the communication circuit in a state in which the operation mode of the frequency adjustment circuit is designated as the second mode.