1. A tire shape inspection method that inspects a sidewall surface of an inspection tire for shape defects by using an image of a sidewall surface of a sample tire having uneven marks in the sidewall surface thereof, the tire shape inspection method comprising: a teaching operation process including 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 tire, and generating a mask image showing positions of the boundary lines, and a height offset image generating step of removing, from the sample original image, regions corresponding to the positions of the boundary lines shown in the mask image, and generating a height offset image by expressing heights of the remaining regions using one or a plurality of offset values; and an inspection operation process including a subtraction processing step of subtracting the height offset image from an inspection image which is a two-dimensional image of the sidewall surface of the inspection tire, and removing boundary regions shown by the mask image, and a shape defect detecting step of inspecting the sidewall surface of the inspection tire for shape defects on the basis of an unevenness-removed image obtained as a result of the subtraction processing step, wherein the height offset image generating step generates, in the sample original image, an offset profile that approximates a base surface which is a sidewall surface having no uneven marks, extracts the uneven marks from the sample original image on the basis of the generated offset profile, and sets heights of the extracted uneven marks as the offset values.