PURPOSE: To prevent the paint-out and omission of characters generated at the time or reducing character pictures and to generate identifiable conversion pictures by reducing the disappearance of thin lines and thickening of the thin lines generated at the time of reducing binary pictures such as facsimile pictures.

CONSTITUTION: This device is constituted of a binary picture memory 11 for storing binary picture data, a picture element block extraction part 12 for the binary pictures for extracting the picture element block of a prescribed size including a picture element of interest and the peripheral picture elements at the time of scanning the picture elements stored in the binary picture memory 11 in a main scanning direction, a thinning processing part 13 for performing a thinning processing on the picture element block and the peripheral picture elements extracted by the picture element block extraction part 12, a changed picture element memory 14 for storing the output of the thinning processing part 13, a logic part 15 for ANDing the binary picture element data of the picture elements related to the thinning processing inside the picture element block extracted by the picture element block extraction part 12 for the binary pictures and the binary picture element data of the picture elements tentatively stored in the changed picture element memory, a reduction processing part 16 and a reduced picture memory 17.