A line position calculating method is a method for calculating a position, in a second direction, of a line along a first direction in an image, based on image data of the image constituted by a plurality of pixels arranged in a matrix form in the first direction and in the second direction, the method obtains pixel data of each of a plurality of pixels arranged in the first direction, calculates an extent of unevenness of each tone value indicating the pixel data of each of the plurality of pixels arranged in the first direction, sets a threshold value according to the extent of unevenness, sets a range based on the threshold value, detects a defective pixel in which the tone value indicating the pixel data is out of the range, calculates a representative value indicating a tone value of a pixel row, based on the pixel data of the pixels other than the defective pixel of the plurality of pixels constituting the pixel row, and calculates a position, in the second direction, of the line, based on the representative value of each pixel row.