To provide a post-treatment method for a UASB apparatus which can sufficiently demonstrate treatment effects by an ascending current anaerobic treatment apparatus and enable the high efficiency treatment of organic wastewater.

In the post-treatment method for the ascending current anaerobic treatment apparatus, biotreatment wastewater, after being treated by the treatment apparatus, is supplied to a denitrification process provided with a denitrification tank and a nitrification tank to be denitrified, the denitrified water is supplied to a precipitation tank to precipitate/separate sludge, the supernatant is discharged as treated water, part of the precipitated sludge is returned to the denitrification process.