Readout characteristics of a near-field optical disk with an acute-edged metallic nano-aperture were investigated. The electromagnetic field distributions of the near field around the aperture were analyzed, and the far-field scattering of the waves transmitted through the phase change disk was calculated by finite-difference time-domain method into which motion equations of free electrons were incorporated. Adjusting the edge angle of the aperture increased the field intensity of near-field light. The influence of the edge angle on output through the recorded mark was also investigated. It was found that there is an optimum edge angle that differs depending on the aperture width.