GPU (graphics processing unit) has a lot of simple numerical computation cores with parallel operation, and the GPGPU (general purpose computing on GPU) is one of the candidates to improve computation efficiency. In this paper, we numericaly analyze the 1D electromagnetic wave propagation based on the CIP method by using the GPU. The computation times are evaluated by changing the thread size per one block and the results shows that the thread size is chosen to be larger than 64 for efficient computation.