An image processing device includes: an estimated size setting section (ACS determination section 151) which sets a document size of a document sheet as an estimated size; a threshold deciding section (ACS determination section 151) which decides a first threshold according to the estimated size; and an ACS determination section (151) which determines that the document sheet is a color document sheet when a number of color pixels reaches the first threshold. When an actual size of the document sheet is smaller than the estimated size, the threshold deciding section decides a second threshold which is smaller than the first threshold, and the document sheet is determined to be a color document sheet when the number of color pixels is the second threshold or greater and is determined to be a black-and-white document sheet when the number of color pixels is smaller than the second threshold.