We experimentally demonstrated the scalability of the terahertz-wave parametric oscillator by using a pump beam with a wide aperture and a high pulse energy. Terahertz-wave absorption by the LiNbO3 crystal in the oscillator is substantially suppressed by employing a surface-emitting cavity configuration. We also improved the conversion efficiency by increasing the parametric interaction in the noncollinear phase-matching geometry. A pump depletion of 54% and a conversion efficiency of 0.9 × 10-6 are achieved. A maximum terahertz output of 382 nJ/pulse was achieved at 1.46 THz using a 8.0-mm-diameter pump beam with a pulse energy of 465 mJ/pulse