PURPOSE: To provide the image data transmission method capable of surely detecting the transmission error.

CONSTITUTION: Image data are divided into blocks by a block dividing circuit 22. Image data divided into blocks are, orthogonally converted at every block by an orthogonal intersecting conversion circuit 26 and is encoded by encoding circuits 36 and 38 and is transmitted by a communication equipment 44. Image data of each block are averaged by an averaging circuit 24 and is reversedly encoded by a reversible encoding circuit 34 and is transmitted. On the reception side, a block average value is calculated from restored image data and is compared with the block average value transmitted after reversible encoding. If they have a large difference, the transmission error is discriminated.