1. An optical condition design method for a compound-eye imaging device comprising: multiple optical lenses for collecting lights from a target object at different angles, and a solid-state imaging element having multiple pixels for receiving lights collected by the optical lenses to image unit images, the optical condition design method being for designing optical conditions of the optical lenses, and comprising: a plane setting step for setting at least one imaginary object plane in front of the optical lenses; a pixel observation point calculating step for changing a part of optical conditions of the optical lenses as variables and calculating positions of points (hereafter referred to as pixel observation points) on the at least one imaginary object plane where lights coming from the pixels of the solid-state imaging element and back-projected through the optical lenses intersect the at least one imaginary object plane when the part of the optical conditions is changed as the variables; an evaluating step for evaluating dispersion in position of the pixel observation points on the at least one imaginary object plane which are calculated in the pixel observation point calculating step; and a condition determining step for determining, as an optimum optical condition of the optical lenses, a set of values of the variables giving a maximum dispersion of the calculated pixel observation points which is evaluated in the evaluating step.