An authentication system determines if a counterfeit ineligible unit is installed in a main device. When connected with a battery pack, a notebook PC generates and combines a random number and a function determination signal using a signal combining unit and transmits same to the battery pack. A first function calculation unit calculates the function of the random number. The battery pack has a signal separation unit to separate the combined signal into the random number and function determination signal, and a second function calculation unit to calculate the function of the random number for transmitting back to the notebook PC. A comparison unit compares the calculation results by the first and second function calculation units to determine whether the connected battery pack is an authorized one and denies connection if the pack is an unauthorized one.