Abstract: There is disclosed a process for nitrifying denitrifying organic waste water to nitrify and denitrify the nitrogenous components in the waste water, which comprises making use of a microbe carrier composed of an organic thermoplastic high molecular compound (thermoplastic polyurethane) in both a nitrifying step and a denitrifying step, preferably in an alternate and repeated manner, wherein the compound has, in a state of complete swelling in water, a water absorption rate in the range of 50 to 3500%, a compressive yield stress of at least 1.0 MPa each being defined by a specific formula, and a particle diameter in the range of 1.0 to 30 mm. The process is capable of efficiently performing nitrifying and denitrifying steps, steadily achieving high treatment performances, contriving compactification of the treatment facility, and curtailing the treatment time and treatment cost, while the microbe carrier can withstand long-term service without any trouble.