Since Urata et al. reported on the Smith Purcell superradiance, numerous studies have been carried out to develop novel type of terahertz free electron lasers. The particle-in-cell finite-difference time-domain (PIC-FDTD) method has been widely employed to numerically study the process. We show our studies on the parallel computing based on the general purpose computation on the graphic processing unit (GPGPU) to accelerate our homemade PIC-FDTD simulation. We have succeeded in reducing the computational time to the quarter of that required for the same simulation using only CPU.