A method of converting audio data encrypted for anti-piracy and copyright protection to a suitable format so that the data can be played in a portable device. A computer first decrypts the encrypted audio data and inserts information for setting equalizer, then encrypts the audio data lightly with the information for setting equalizer and transmits the data to a portable device. The portable device sets equalizer suitable to the lightly-encrypted audio data using the information for setting equalizer inserted by the computer, then decrypts the lightly-encrypted audio data, and outputs the decrypted audio data while amplifying or attenuating audio data for each band according to the set equalizer. Therefore, heavily encrypted audio data can be playable in a portable device with relatively low computing power and, by setting equalizer mode suitable to a music genre, the audio data can be conveyed to a listener authentically.