1. A method for detecting a two-dimensional code of three position detection patterns provided in positions corresponding to respective apices of a right angled isosceles triangle, and a data region is provided in a region defined by the position detection patterns, the method comprising: detecting a plurality of position detection patterns included in an image; identifying positions of the center points of the detected position detection patterns; detecting groups of three center points, the three center points of each group constituting apices of a corresponding right angled isosceles triangle; detecting, from among the groups of three center points, combinations of three center points, the three center points of each combination being included in a corresponding same two-dimensional code from among the identified center point positions, by defining a candidate two-dimensional code region as a square formed by a combination of three center points, setting a virtual line that forms a square that is separated by a predetermined distance from an outer periphery of the candidate two-dimensional code region, and determining that a previously standardized blank region exists along the virtual line if the proportion of white pixels to the total number of pixels on the virtual line exceeds a threshold, and determining the combination of three center points that forms the candidate two-dimensional code region with the virtual line having the blank region being included in the corresponding same two-dimensional code; and determining the candidate two-dimensional code region in the combination of three center points included in the corresponding same two-dimensional code as a two-dimensional code region to be decoded.