1. An eye margin calculating apparatus that calculates a margin between an eye pattern generated by superimposing waveform signal pieces with a specified length cut out from a waveform signal generated by a simulator and a mask indicating the size of an opening required for the eye pattern, the eye margin calculating apparatus comprising: first calculating means for calculating a center location of the eye pattern; second calculating means for calculating time coordinate values and voltage coordinate values of feature points included in the mask in a case that a center location of the mask is arranged at the center location of the eye pattern; third calculating means for calculating a margin in a voltage axis direction based on voltage coordinate values of first feature points and voltage coordinate values of waveform signal piece parts associated with the first feature points, the first feature points being feature points not on a time axis among the feature points; and fourth calculating means for calculating a margin in a time axis direction based on time coordinate values of second feature points and time coordinate values of waveform signal piece parts associated with the second feature points, the second feature points being feature points on the time axis among the feature points.