Improvement of the in-plane spatial resolution in cross-sectional imaging using terahertz waves was investigated. The effects of frequency filtering, which limits the frequency range of the terahertz waves, and unsharp mask processing, based on a two-dimensional convolution of the imaging result and a mask consisting of a linear combination of the original image and a Laplacian filter, were evaluated. The filtering methods were successful in improving the spatial resolution up to the sampling limit of 0.4 mm. The combination of frequency filtering and unsharp mask processing was found to be effective for improving the contrast in addition to the spatial resolution. Frequency filtering, unsharp mask processing, and their combination were applied to terahertz wave cross-sectional imaging of an IC card.