An apparatus for treating a radioactive nitrate waste liquid includes: a denitrification tank (12) that accommodates active sludge which adsorbs or takes in a radioactive substance in a nitrate waste liquid (11) containing nitrate and the radioactive substance and in which an anaerobic microorganism that reduces the nitrate to nitrogen gas grows; a reaeration tank (14) that aerates and mixes a denitrification-treated liquid (24) treated in the denitrification tank (12) with the active sludge in which the aerobic microorganism grows; and a sludge dissolution tank (81) that dissolves redundant sludge (26A, 26B) discharged from the denitrification tank (12) and the reaeration tank (14). Acetic peracid (80) is supplied to the sludge dissolution tank (81) to dissolve redundant sludge, a sludge lysate is supplied to the denitrification tank (12) as a carbon source (22), and acetic acid is supplied to the denitrification tank (12).