Abstract: An apparatus for treating waste water in which a large number of soft ribbon-like contact members having loop-like fluffs for retaining the active sludge are vertically spread between a large number of suspension rods arranged vertically inside the catalytic oxidation tank in order to permit the ribbon-like contact members to retain the active sludge, to promote the biological oxidation reaction and to permit the catalytic oxidation liquid to flow through the gaps between the implanted ribbon-like contact members that are capable of swaying, thereby preventing clogging of the passages due to the active sludge. Both ends of each of said large number of suspension rods are interconnected by bendable ribbon-like gap-retaining members.