We propose and numerically investigate integrated photonic crystal waveguides (PhC-WGs) formed in a semiconductor slab to realize an ultrasmall and highly efficient terahertz (THz) wave source. The structure consists of a straight PhC-WG with low-group-velocity and low-dispersion (LVLD) for efficient difference frequency generation (DFG) connected to two PhC-WGs to introduce two fundamental lights into the LVLD PhC-WG. The fundamental light propagating through each PhC-WG designed to enhance their electric fields by the slow-light effect is efficiently coupled to the LVLD PhC-WG owing to the reduced refractive index differences at the boundaries of the heterostructures. The DFG from the two fundamental lights was numerically simulated, and a temporal intensity oscillation corresponding to the difference in frequency was clearly observed. By comparing the DFG intensities of the integrated structures with an LVLD PhC-WG and a strip WG, the estimated DFG intensity from the LVLD PhC-WG was more than 100 times higher than that from the strip WG. These results indicate the effectiveness of the proposed heterostructure in the application of a highly efficient THz source with an ultrasmall footprint compared with conventional materials.