There is provided a process for purification treatment of wastewater containing an organic substance. There is provided a process for purification treatment of wastewater containing an organic substance by carrying out supercritical treatment or subcritical treatment, which comprises subjecting the wastewater to at least one solid-liquid separation pretreatment selected from separation treatment with a flocculant, sedimentation treatment, flotation treatment and filtration treatment; and then subjecting the wastewater to primary treatment as biological treatment; and subsequently purifying the wastewater by treatment including secondary treatment as supercritical treatment or subcritical treatment.