1. An image processing apparatus that performs binarization processing of multi-valued image data using an error diffusion method so as to output binary image data to an image formation apparatus, the image processing apparatus comprising: an image processor that performs error diffusion processing by applying a predetermined error diffusion matrix to multi-valued image data having pixels two-dimensionally arranged in a main scanning direction that is a direction in which heads are arranged and in a subscanning direction orthogonal to the main scanning direction, so as to convert the multi-valued image data into binary image data, wherein the image processor applies, as the error diffusion matrix, an error diffusion matrix in which a diffusion coefficient of a pixel diagonal to a focused-on pixel is greater than diffusion coefficients of pixels other than the pixel diagonal to the focused-on pixel.