An apparatus (10A; 10B; 10C; 10D) for treating a radioactive nitrate waste liquid, comprising: a denitrification tank configured to accommodate active sludge which adsorbs or takes in a radioactive substance in a nitrate waste liquid containing nitrate and the radioactive substance and in which an anaerobic microorganism that reduces the nitrate to nitrogen gas grows; a reaeration tank configured to aerate and mixe a denitrification-treated liquid treated in the denitrification tank with the active sludge in which the aerobic microorganism grows; and a sludge dissolution tank configured to dissolve redundant sludge; a redundant-sludge supplying line configured to supply part of the redundant sludge (26A, 26B) from the denitrification tank and the reaeration tank to the sludge dissolution tank; said apparatus (10A; 10B; 10C; 10D) for treating a radioactive nitrate waste liquid being configured to supply a sludge solvent (80, 83) to the sludge dissolution tank to dissolve the redundant sludge, and to supply a sludge lysate to the denitrification tank as a carbon source; characterized in that the denitrification tank comprises: a pH adjusting unit that supplies a pH adjuster used for adjusting pH of the nitrate waste liquid; a first solid-liquid separating unit that separates a denitrified liquid treated with the active sludge into a solid content containing sludge and a denitrification-treated liquid; and a gas supplying unit that is arranged on a lower side of the first solid-liquid separating unit and supplies gas not containing oxygen into the denitrification tank, and the reaeration tank comprises: a second solid-liquid separating unit that further separates the denitrification-treated liquid treated with the active sludge into redundant sludge and a treated liquid; and an air supplying unit that is arranged on a lower side of the second solid-liquid separating unit and supplies air into the reaeration tank. a denitrification tank configured to accommodate active sludge which adsorbs or takes in a radioactive substance in a nitrate waste liquid containing nitrate and the radioactive substance and in which an anaerobic microorganism that reduces the nitrate to nitrogen gas grows; a reaeration tank configured to aerate and mixe a denitrification-treated liquid treated in the denitrification tank with the active sludge in which the aerobic microorganism grows; and a sludge dissolution tank configured to dissolve redundant sludge; a redundant-sludge supplying line configured to supply part of the redundant sludge (26A, 26B) from the denitrification tank and the reaeration tank to the sludge dissolution tank; said apparatus (10A; 10B; 10C; 10D) for treating a radioactive nitrate waste liquid being configured to supply a sludge solvent (80, 83) to the sludge dissolution tank to dissolve the redundant sludge, and to supply a sludge lysate to the denitrification tank as a carbon source; a pH adjusting unit that supplies a pH adjuster used for adjusting pH of the nitrate waste liquid; a first solid-liquid separating unit that separates a denitrified liquid treated with the active sludge into a solid content containing sludge and a denitrification-treated liquid; and a gas supplying unit that is arranged on a lower side of the first solid-liquid separating unit and supplies gas not containing oxygen into the denitrification tank, and a second solid-liquid separating unit that further separates the denitrification-treated liquid treated with the active sludge into redundant sludge and a treated liquid; and an air supplying unit that is arranged on a lower side of the second solid-liquid separating unit and supplies air into the reaeration tank.