To reduce the data quantity representing a black part in the case of sending image data including many blank parts.

The transmitter is provided with a black block detection circuit 1 that detects a blank of image data and provides an output of a signal denoting a line number of a black and expresses the blank by a line number itself. A data compression circuit compresses data and a selector 4 is used to send compression data or a blank line number properly selectively. A white line data generating circuit 7 at a receiver side generates a blank in response to a blank line number. A data expansion circuit 8 expands data for the part being not a blank and a selector 9 selects properly white line data or expanded image data.