The in-plane spatial resolution of time domain terahertz imaging using broadband pulsed terahertz waves (0.1-1.6THz) was evaluated using the edge of a metal plate. The spatial resolution of 0.8mm could be improved to 0.4mm by limiting the frequency region of the broadband terahertz wave to 1.0-1.5THz. Experiments using a resolution target showed that the method is effective for improving the spatial resolution and the constrast ratio.