Even when a combination of the stegonography technique and prediction encoding is applied to sound encoding, a sound encoder does not cause deterioration in quality of decoded signals. In the device, an encoding section (102) outputs an encoding code (I) to a bit embedding section (104). A function extension encoding section (103) generates an encoding code (J) for information required for extending functions of the sound encoder (100) and outputs it to the bit embedding section (104). The bit embedding section (104) embeds information on the encoding code (J) into a part of bits of the encoding code (I) and outputs the resultant encoding code (I′). A synchronization information generating section (106) generates synchronization information according to the encoding code (I′) after the bit embedding and outputs the synchronization information to the encoding section (102). The encoding section (102) updates the internal state and the like on the basis of the synchronization information and encodes the next digital sound signal (X).