Data communication apparatus

A data communication apparatus wherein the stealthiness has been enhanced by significantly increasing the time required for a wiretapper to decrypt an encrypt text. The data communication apparatus is constituted by connecting a data transmitting apparatus (10101) and a data receiving apparatus (10201) via a transmission path (110). The data transmitting apparatus (10101) receives a first predetermined initial value (key information) and information data, generates a multi-level signal the signal level of which varies substantially like a random number, and converts the multi-level signal to a modulated signal of a predetermined modulation format for transmission. The data receiving apparatus (10201) demodulates the modulated signal to output the multi-level signal, and then reproduces the information data from the multi-level signal and a second predetermined initial value (key information) that are received.