Provided is technology enabling reliably erasing information on recovered paper, and whitening (reducing blackening) of the sheet. A sheet processing device has an image inspector configured to determine from a sheet printed with image information at least one of a background color of the sheet, and position, size, and color of the image information on the sheet; an image analyzer configured to determine, based on the evaluation result from the image inspector, a type of ink masking agent and an amount of ink masking agent; an ink masking agent applicator configured to apply to the image information printed on the sheet an ink masking agent of the specific type and specific amount determined by the image analyzer; and a shredder configured to comminute the sheet to which the ink masking agent was applied by the ink masking agent applicator.