The first known demonstration of tunable terahertz-wave generation by difference-frequency generation of dual signal-wave quasi-phase-matched optical parametric oscillation was performed with periodically poled LiNbO(3) (PPLN) with a series of gratings. An organic ionic salt, 4-dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST), was used as a nonlinear crystal. A compact terahertz-wave source resulted, and changing the temperature of the PPLN permitted the wavelength to be varied from 120 to 160 microm. The wavelength could be tuned from 100 to 700 microm by proper selection of combinations of periodically poled gratings.