The present invention provides a used paper diaper processing method comprising: a swell suppressing step for immersing a used paper diaper in a swell suppressant solution to suppress the swelling of the polymer with high water absorbability; a breakup step for breaking up the paper diaper subsequent to the swell suppressing step while stirring the same immersed in a processing liquid, into a cover material breakup substance and an absorbing material breakup substance; a cover material breakup substance removing step for removing the cover material breakup substance; a filtering step for filtering the processing liquid containing the absorbing material breakup substance and excreta to separate and remove the absorbing material breakup substance; and a liquid discharging step for discharging the processing liquid containing excreta into a sewage treatment facility; and an absorbing material breakup substance recovering step for recovering the absorbing material breakup substance separated and removed by the filtering step.