We propose a microwave/millimeter-wave signal generation device using difference frequency generation in a rectangular waveguide embedded with a nonlinear LiTaO₃ crystal. The rectangular waveguide is designed as a single-guided mode structure for the generated microwave/millimeter-wave signal. Dispersion of the rectangular waveguide is also utilized to obtain a phase matching condition between the input lightwaves and the generated microwave/millimeter-wave. In the experiment, a 25GHz signal was successfully generated. Microwave characteristics of the proposed device were also investigated by using a 3-D electromagnetic wave analysis software. We expect the proposed device is applicable for higher-frequency microwave/millimeter-wave signal generation.