A terahertz-wave generating device including an optical waveguide containing an electrooptic crystal includes: first and second optical waveguides through which first and second light beams respectively propagate; a propagation portion through which a first terahertz wave propagates, the first terahertz wave being generated from the second optical waveguide in a direction different from a direction of the second light beam; and a delay portion arranged at incidence sides of the first and second light beams and configured to delay the first light beam relative to the second light beam. The first optical waveguide and the second optical waveguide are arranged with the propagation portion interposed therebetween. A first equiphase surface of the first terahertz wave is substantially aligned with a second equiphase surface of a second terahertz wave generated from the first optical waveguide in a direction different from a direction of the first light beam.