1. A computer-readable medium encoded with a program of instructions executable by a computer to perform a method for reading a two-dimensional code with digital information expressed by data marks displayed on a plurality of cells arrayed two-dimensionally, comprising: detecting from an image data containing the two-dimensional code, an image pattern that is distinguished from surrounding cells by verifying an image on the image data with a verification pattern, the detected image pattern includes the image of one or more data marks and each data mark is surrounded by a blank area in each cell, wherein the detecting of the image pattern includes: verifying the image on the image data with a verification pattern while scanning the image data; and determining the verified image according to a determining formula, and wherein: the verification of the image on the image data is performed using a verification pattern consisting of at least two image areas defined according to assumed pixel tones; and the determining of the image on the image data is performed by assigning pixel tone values calculated in correspondence to each of the image areas of the verification pattern in the determining formula.