An apparatus for controlling Transmit (Tx) power in a human body communication system, the apparatus comprising: a voltage manager adapted to change the Tx power of a portable terminal according to a distance between the portable terminal and a counterpart portable terminal, wherein the portable terminal and the counterpart portable terminal are configured to communicate via the human body, wherein the voltage manager is configured to: measure a current flowing between the portable terminal and the counterpart portable terminal, amplify an analog signal which contains information on an output voltage that is based on the current that varies according to a distance between an electrode of the portable terminal and an electrode of the counterpart portable terminal, compare the amplified output voltage with a predefined threshold, determine whether the Tx power needs to be changed based on a result of the comparison, and change the Tx power according to a result of the determination, and the distance between the electrodes corresponding to a distance between body parts, on which the portable terminal and the counterpart portable device are brought in contact, or a distance between electrodes based on the number of users performing human body communication. a voltage manager adapted to change the Tx power of a portable terminal according to a distance between the portable terminal and a counterpart portable terminal, wherein the portable terminal and the counterpart portable terminal are configured to communicate via the human body, wherein the voltage manager is configured to: measure a current flowing between the portable terminal and the counterpart portable terminal, amplify an analog signal which contains information on an output voltage that is based on the current that varies according to a distance between an electrode of the portable terminal and an electrode of the counterpart portable terminal, compare the amplified output voltage with a predefined threshold, determine whether the Tx power needs to be changed based on a result of the comparison, and change the Tx power according to a result of the determination, and the distance between the electrodes corresponding to a distance between body parts, on which the portable terminal and the counterpart portable device are brought in contact, or a distance between electrodes based on the number of users performing human body communication. measure a current flowing between the portable terminal and the counterpart portable terminal, amplify an analog signal which contains information on an output voltage that is based on the current that varies according to a distance between an electrode of the portable terminal and an electrode of the counterpart portable terminal, compare the amplified output voltage with a predefined threshold, determine whether the Tx power needs to be changed based on a result of the comparison, and change the Tx power according to a result of the determination, and the distance between the electrodes corresponding to a distance between body parts, on which the portable terminal and the counterpart portable device are brought in contact, or a distance between electrodes based on the number of users performing human body communication.