To provide an anaerobic wastewater treatment apparatus hardly generating calcium-derived sludge waste and capable of suppressing scale generation, when performing anaerobic organism treatment of organic wastewater containing high-concentration calcium.

This anaerobic wastewater treatment apparatus 1 treating organic wastewater containing calcium of 250 mg/L or above includes: an anaerobic reaction tank 10 for performing anaerobic organism treatment; a pH adjusting agent tank 12 as a pH adjustment means adjusting pH inside the anaerobic reaction tank 10 to 8 or below; and an electrical conductivity adjusting agent tank 14 as an electrical conductivity adjustment means adjusting electrical conductivity inside the anaerobic reaction tank 10 to 12,000 μS/cm or above.