An image data transmission system performs a coding of a plurality of picture elements in units of blocks. In order to prevent a deterioration of an image quality caused due to a loss of a packet cell on a transmission line without increasing a volume of data to be transmitted, a unit for producing a signal designating a loss compensation method to be performed on a receiving side is provided to be performed on a transmission side. The receiving side provides a unit for detecting a loss of the packet or cell and a unit for compensating the lost block data by the loss compensating method designated by the transmission side.