1. A two-dimensional code reader comprising: an image acquisition unit acquiring an image formed by shooting an image of a region including a plurality of two-dimensional code regions; a component conversion unit converting the image acquired by the image acquisition unit into an edge image indicating the distribution of edge components included in the image; a region extraction unit extracting a region formed in conformity with a predetermined region rule that defines an outer shape formed with an outline portion of each of the two-dimensional code regions and also defines a shape set as a shape similar to the outer shape, as a candidate region each containing the corresponding two-dimensional code region, from the edge image converted by the component conversion unit; a candidate-region determination unit determining whether the candidate region extracted by the region extraction unit is a valid candidate region on the basis of shape information that is information in which a particular error range is set with respect to a parameter of an ideal outer shape being the outer shape formed with the outline portion of the two-dimensional code region; a pattern determination unit determining, for respective corresponding regions that correspond to the candidate region in the image acquired by the image acquisition unit that has been determined to be valid by the candidate-region determination unit, whether a characteristic pattern that is a pattern for determining whether a pattern formed in the corresponding region is the two-dimensional code region is included in elements that form the corresponding region; a region detection unit detecting the corresponding region determined to include the characteristic pattern by the pattern determination unit, as the two-dimensional code region; an information identifying unit identifying information indicated by the two-dimensional code region, for each of the two-dimensional code regions detected by the region detection unit; a repetitive execution unit successively allowing conversion of an image into an edge image by the component conversion unit for different kinds of conversion conditions and also repeatedly allowing, for each of the edge images converted for the respective conversion conditions, extraction of a candidate region by the region extraction unit, determination of a valid candidate region by the candidate-region determination unit, determination of a characteristic pattern by the pattern determination unit, and detection of a two-dimensional code region by the region detection unit; and an agreement determination unit determining whether each candidate region extracted by the region extraction unit for a particular edge image and determined to be valid by the candidate-region determination unit agrees with a candidate region (hereinafter referred to as “already-extracted candidate region”) extracted by the region extraction unit for another edge image and determined to be valid by the candidate-region determination unit, based on both of the candidate regions, the candidate-region determination unit determining that the extracted candidate region is a valid region when information corresponding to the shape information of the candidate region extracted by the region extraction unit is included within the error range, and determines that the extracted candidate region is an invalid region when information corresponding to the shape information falls outside the error range, and the repetitive execution unit allowing the determination by the pattern determination unit as to whether a characteristic pattern is included and the detection of a two-dimensional code region by the region detection unit, only for a candidate region that has been determined by the agreement determination unit not to agree with the already-extracted candidate region, among candidate regions extracted by the region extraction unit and determined by the candidate-region determination unit to be valid.