Nitride semiconductor is a material having potentials for realizing wide frequency range of quantum-cascade lasers (QCLs), i.e., 3～20 THz and 1～8 μm, including an unexplored terahertz frequency range from 5 to 12 THz, as well as realizing room temperature operation of THz-QCL. The merit of using an AlGaN-based semiconductor is that it has much higher longitudinal optical phonon energies (ELO> 90meV) than those of GaAs-based semiconductors (~ 36 meV). In this study, we demonstrate the first lasing action of GaN-based QCLs. We introduced an unique quantum design active region, i.e., “pure 3-level system design”, which is consisting of 2 quantum wells (QWs) per one period. We grew GaN/AlGaN QC structures by using molecular beam epitaxy (MBE). The layer structure of the GaN/AlGaN QCL was consisting of 100~200 periods of QC active layers sandwiched by Si-doped (Al)GaN upper and lower contact layers, which were grown on a high-quality AlGaN/AlN template grown on a c-plane sapphire substrate. After the crystal growth, we fabricated QCL sample with single metal plasmon waveguide structure. Lasing spectrum was obtained at 5.39 THz measured under pulsed current injection at 5.8K. The threshold current density Jth and the threshold voltage Vth were 1.75 kA/cm2 and 14.5 V, respectively. We also fabricated similar design GaN/AlGaN QCL by metal organic chemical vapor deposition (MOCVD), and obtained lasing at 6.97 THz. The Jth and Vth of the MOCVD grown QCL were 0.75 kA/cm2 and 27 V, respectively, measured at 5.2 K.