With the progress of industrial technologies, need for portable Josephson voltage standard system is increasing. However, widespread use of Josephson voltage standard system is prevented by the necessity of liquid helium in the operation of the system. To overcome the problem, we have been developing niobium nitride(NbN)-based Josephson junctions for voltage standard system. NbN is known as a superconductor having a critical temperature over 15K and its high-quality thin films can be prepared by reactive sputtering without using substrate heating. We recently succeeded to develop NbN-based Josephson junctios (NbN/TiN/NbN junction) which can be operated at 10K. Using this junction, we will construct a liquid-helium-free Josephson voltage standard system within a few years.