Composting of livestock manure is a significant source of greenhouse gases such as nitrous oxide (N2O), but the complexity of the N2O generation pathway hinders efforts to develop effective countermeasures. In this paper, we present one mitigation option for N2O emissions from composting. Nitrite (NO2-) has an important role in N2O generation. It has been observed that significant N2O emissions from composting are induced when NO2- is accumulated during nitrification. Accordingly, in our technique, nitrite-oxidizing bacteria (NOB), as nitrifying bacteria, are added in the middle of composting fermentation to prevent NO2- accumulation. Adding NOB prevents prolonged NO2- accumulation, which had resulted in low N2O emissions. This technique is also cost-effective, because mature compost can be used as an NOB source. Mitigating N2O emissions allows more nitrate nitrogen (NO3-), of high value as fertilizer, to be preserved in the compost product. Moreover, it was confirmed that this technique could be combined with that for NH3 mitigation using chemical reagents. Before actual use, it is necessary to investigate the methods used to determine optimal timing to add NOB source and prevent N2O from being generated from an NO3- reduction.