1. A tyre shape inspection method for inspecting a shape defect in a sidewall surface of a tyre under inspection, by using an image of a sidewall surface of a sample tyre having the sidewall surface in which uneven marks are formed, the method comprising: a teaching operation step; and an inspection operation step, wherein the teaching operation step comprises: a mask image generating step of detecting boundary lines which are contours of the uneven marks, in a sample original image which is a two-dimensional image of the sidewall surface of the sample tyre, and generating a mask image showing positions of the boundary lines; and a height offset image generating step of generating a height offset image which shows a height of the uneven marks, the height offset image being obtained by, in use of a plurality of discrete height threshold values, classifying a height of regions in the sample original image which remain after excluding regions corresponding to the positions of the boundary lines shown in the mask image, and the inspection operation step comprises: a difference processing step of excluding the uneven marks from an inspection image which is a two-dimensional image of the sidewall surface of the tyre under inspection to generate an unevenness-excluded image, by subtracting the height offset image from the inspection image; and a shape defect inspecting step of inspecting the shape defect in the sidewall surface of the tyre under inspection, on the basis of the unevenness-excluded image.