We numerically analyze the generation of near-field light at a 50-nm-wide slit used for two-step photoionization detection of alkali-metal atoms with high spatial resolution, considering the influence of rounding the slit edges. In the case where near-field light is generated via total-internal reflection of s-polarized light introduced along the slit, the intensity decreases with increasing the radius of curvature of the slit edge. Finite-difference time-domain simulations indicate that the electric field is concentrated in the upper left and right corners of the slit when the radius of curvature is small, but the enhancement is resolved due to edging down. Assuming that the intensity of an Ar+ laser beam with a wavelength of 476.5 nm is 5 × 104 W/cm2, the radius of curvature of the edge of less than 15 nm is required for the ionization efficiency exceeding 10 % of slow 87Rb atoms with the incident speed of 10 cm/s. The throughput of near-field light increases with wavelength for the s-polarization, but decreases with wavelength for the p-polarization.