Provided herein is a large caliber terahertz wave generating device having a photonic crystal structure. The device includes a first electrode and a second electrode. The first electrode includes a first line pattern extending in a first direction, second line patterns coupled to the first line pattern and extending in a second direction, and third line patterns which are coupled to the first line pattern, extend in the second direction, are disposed between the second line patterns, and are longer than the second line patterns. The second electrode includes a fourth line pattern which extends in the first direction, fifth line patterns coupled to the fourth line pattern and extending in the second direction, and sixth line patterns which are coupled to the fourth line pattern, extend in the second direction, are disposed between the fifth line patterns, and are longer than the fifth line patterns.