PURPOSE: To make the density of a highlight part of an output picture stable by binarizing a multi-value picture data into binary information of a prescribed density when the density of an input multi-value picture data is decided to a prescribed density or below in a picture forming device of the electronic photography system.

CONSTITUTION: The density of an inputted multi-value picture data A is decided as to whether or not the density is a prescribed density or below by a low density detection circuit 201 of a low density binarizing circuit 200 and a selector 205 is controlled in response to the result of decision and a data of a prescribed density or over is outputted from a selector 205 as it is. On the other hand, when the density is decided to be a prescribed density or lower, the picture element data density is compared with a density average S by a mean density arithmetic circuit 203 and binarized into a binary data 0 of a least contrast density and a binary data 1 representing a prescribed density by a binary processing circuit 202 and outputted and the picture is formed by electronic photography system based on the data and the density of the highlight part of the output picture is formed stably and stored surely, and then the gradation level width is considerably increased.