Digital halftoning is used to quantize a grayscale image to a binary image. Error diffusion halftoning generates a high-quality binary image, but also generates some defects such as the warm effect, sharpening, and so forth. To reduce these defects, Kite proposed a modified threshold modulation method that utilizes a multiplicative parameter for controlling sharpening. Nevertheless, some degradation was observed near the edges of objects with a large luminance change. In this paper, we propose a method of controlling the multiplicative parameter in proportion to the magnitude of the local edge slope. The results of computer simulation show a greater reduction of sharpening in the halftone image. In particular, there is a great improvement in the quality of the edges of objects with a large luminance change.