We developed a compact THz-wave parametric generator (TPG) system with a diode-pumped Nd:YAG laser that was specially designed for pumping the TPG. The pump laser produced an output energy of 11 mJ in a single Q-switched pulse with a pulse width of 6 ns. Using MgO:LiNbO 3 crystals, a THz-wave energy of 1.7 pJ was obtained in unseeded operation, and a broad THz-wave spectrum (1.4-2.2 THz) was then observed. Furthermore, a narrow THz-wave linewidth of less than 33 GHz was achieved by injection seeding of the idler wave. The THz-wave energy was increased by a factor of 3 compared to the unseeded output.