1. A method for a stereographic image generation apparatus for generating a stereographic image, the method comprising: a calculating step of calculating Z-values of each pixel based on only image data for that pixel, the pixels forming an image, each Z-value being assigned to a pixel, and each Z-value representing a depth of an object corresponding to the pixel; an adjusting step of adjusting a Z-value of a target pixel obtained in said calculating step using a Z-value of at least one individual pixel other than the target pixel; and a generating step of determining an amount of displacement of the target pixel on the basis of an adjusted Z-value of the target pixel, and displacing the target pixel horizontally by the determined amount, to generate images for the right and the left eyes, wherein: in said calculating step the Z-value of each target pixel is obtained by adding predetermined weights to color components of image data of the target pixel, and in said adjusting step: the Z-values of each pixel are adjusted so that a single step available for a Z-value of a pixel corresponding to an object located backward in an original image express deeper depth than a single step available for a Z-value of a pixel corresponding to an object located forward in the original image; an average of Z-values of all pixels within an area which includes the target pixel is obtained; the Z-value of the target pixel is replaced by the obtained average; a step size of quantization of the Z-value is determined based on a value of a parameter specified by a user; and either an upper limit or a lower limit of the calculated Z-value is determined based on the value of the parameter specified by the user.