1. A drive method for a stereoscopic image display apparatus, the stereoscopic image display apparatus including: a plane display device having a plurality of pixels arranged on a display face; a variable polarization cell provided in front of the plane display device, the variable polarization cell including a first electrode substrate having n (where n≧1) transparent first electrodes arranged in parallel thereon, a second electrode substrate having m (where m≧1) transparent second electrodes arranged in a direction substantially perpendicular to the direction in which the first electrodes are arranged, and liquid crystal sandwiched between the first electrode substrate and the second electrode substrate, wherein a polarization direction of light rays from the plane display device is made variable according to a voltage applied between the first and second electrode substrates; and an optical plate provided on an opposite side of the variable polarization cell from the plane display device, the optical plate including a transparent substrate, a lens substrate having a plurality of cylindrical lenses arranged thereon so as to have major axes in the same direction, and a double refraction substance inserted between the transparent substrate and the lens substrate, wherein a major axis direction of the double refraction substance is substantially parallel to major axis directions of lenses of the lens substrate, a minor axis direction of the double refraction substance is substantially perpendicular to the major axis directions of the lenses of the lens substrate, and light rays from the pixels obtained via the variable polarization cell are assigned to predetermined angles, the stereoscopic image display apparatus being capable of changing over between a three-dimensional image and a two-dimensional image and displaying the image, when displaying the three-dimensional image and the two-dimensional image in a background and a window on a same display plane, the background with one of the three-dimensional image and the two-dimensional image and displaying a first to pth (where p≧1) windows with the other image, the drive method comprising: setting an ith flag bit (where 1≦i≦p) concerning the first electrodes and indicating whether an ith window overlaps the first electrodes, and setting an ith flag bit concerning the second electrodes and indicating whether the ith window overlaps the second electrodes; dividing one frame period of the plane display device into p sections as first and second pulses of voltage to be applied respectively to the first and second electrodes, preparing waveforms of 2 p kinds which differ in at least one value of the first and second pulses in the section obtained by the division, and associating the waveforms with a set of values of the first to pth flag bits concerning the first and second electrodes; selecting one first electrode and one second electrode respectively from the n first electrodes and the m second electrodes; and applying the first and second pulses associated with the set of values of the first to pth flag bits concerning the selected first and second electrodes to the selected first and second electrodes.