To measure the pretilt angle in a liquid crystal cell, the crystal rotation method is commonly used because it is very precise and the measurement time is short. However, if the cell gap is not uniform, a significant error occurs due to a shift of the measurement point. This shift is caused by refraction at the glass plate when rotating the cell. In this paper, we estimate the magnitude of this error theoretically and experimentally and propose an improvement of the crystal rotation method by which an accurate value of the pretilt can be obtained even when the cell gap is not uniform.