In order to shorten the time required for encryption and decryption of communication data, the contents of input data are analyzed by a discrimination method such as pattern matching, and in accordance with this analysis result, it is checked whether the received data is particular data. A portion of data judged as the particular data is encrypted and sent to an output buffer, whereas a portion other than the particular portion is not encrypted but is directly sent to the output buffer. Thereafter, the contents in the output buffer are transmitted. The particular data includes a control code of print data, upper bits of image data, predetermined discrete bits of voice data, a conversion tale for compression data, and the like.