PURPOSE: To develop anaerobic treatment at an early stage to stably perform the same thereafter by providing a structure having anaerobic bacteria immobilized thereon by a polymer gel in an anaerobic treatment tank.

CONSTITUTION: A structure having obligate or facultative anaerobic bacteria immobilized thereon by alginic acid or acrylic acid gel is received in an anaerobic treatment tank treating excretion, sewage or industrial waste water and the alginic acid or acrylic acid gel is degraded by the phosphate ion contained in waste water to expose or liberate said bacteria and treatment function is developed at an early stage to perform stable treatment thereafter.