A washing method of red mud that is produced when an alumina-containing ore having a goethite to hematite mass ratio of 60/40 (goethite/hematite) or more in the iron component thereof is heat-treated with an alkaline solution at a temperature of 110° C. to 160° C. to obtain an aluminate liquor, including: mixing the red mud with a diluting fluid to obtain suspension that has an aluminum concentration of 95 g/L or less in terms of Al2O3; adding a flocculant to the suspension, wherein, the flocculant includes a polymer having a repeating unit derived from (meth)acrylic acid or a salt thereof and being substantially free from a repeating unit derived from vinylhydroxamic acid compound or a salt thereof; and settling and separating the red mud so as to wash the red mud.