To develop an efficient local oscillator (LO) integrated with a SIS mixer operating above the gap frequency of Nb, we have investigated the radiation power P of NbN-based flux-flow-type Josephson oscillators (FFO's). The designed and fabricated chip incorporates FFO's, SIS power detectors (DET's), and their coupling circuits. Both FFO's and DET's consist of epitaxial NbN/AlN/NbN junctions with high critical current density JC (15C<78 kA/cm2). The most part of the coupling circuit consists of NbN/SiO2/Al microstrip lines whose rf-loss is approximately 3 dB. It has been found P>200 nW, enough for the optimum pumping of a SIS mixer with rf-resistance of 50 Ω, is coupled to DET's for 0.5-0.9 THz. The coupling bandwidth is larger than 20% of its central frequency. In the band, the radiation frequency is tuned by the control current through the FFO of 10-100 mA. The maximum coupling frequency of the present experiment is quantitatively agreed with the theoretical one. The peak power of 1.3 μW is detected at 0.76 THz. The dissipated power in a FFO is smaller than 500 μW, which is 10-4 times less than that of semiconductor sources. These results indicate the feasibility of NbN-based FFO's for a practical on-chip LO.