The finite difference time domain method (FDTDM) is a robust numerical scheme for time-dependent electromagnetic wave propagation phenomena that uses orthogonal meshes, like staggered meshes, also known as Yee lattices. However, treating complex shaped domains is challenging for the FDTDM. Meshless methods, in contrast, do not require meshes for a geometrical structure. The meshless time domain method (MTDM), based on the radial point interpolation method, can be used for numerical simulations in computational electromagnetics. In MTDM, shape functions have to be generated before the time-dependent calculation, and the computational cost involved can be very large. We herein propose a new method for reducing the computational cost of generating shape functions and we confirm the effectiveness of the proposed method by numerical experiments.