We have developed a hot electron bolometer mixer (HEBM) at 3.1 THz with a terahertz quantum cascade laser (THz-QCL) as a local oscillator. Both the HEBM and the THz-QCL device are fabricated in our laboratory. NbN film with 3 nm thickness was deposited on MgO substrate. The output power of the metal-metal waveguide type 3.1 THz-QCL was about 100 μW at a heat sink temperature of 15 K. The lowest uncorrected receiver noise temperature of the HEBM was 1200 K (DSB). The corrected receiver noise temperature was 810 K (DSB). The measured IF noise bandwidth was about 3 GHz. The spectroscopic Allan time was 10 s. We have also demonstrated detection of emission line spectra of methanol [CH3OH].