1. An image conversion parameter calculation device comprising: a pixel selection unit which randomly selects pixels of a predetermined number or pixels of not more than the predetermined number from a first image; and a parameter derivation unit which derives a conversion parameter concerning the selected pixels selected by the pixel selection unit, the conversion parameter being a parameter for converting, to the first image, a second image that is subject to image alignment with the first image, by repeatedly performing as follows, until a conversion parameter update becomes to be smaller than a first threshold; calculating the conversion parameter update by calculating, for each selected pixel, a product of a transposed matrix of a sub-product of derivative for each selected pixel and a Jacobian matrix in each of the selected pixels, and an error between pixel value at a converted pixel location when converting the pixel in the second image corresponding to the selected pixel and pixel value of the selected pixel in the first image, and calculating a sum of the products, and updating the conversion parameter, by composition of the conversion parameter for converting the pixel in the second image with the conversion parameter update, wherein the pixel selection unit includes: a pixel extraction unit which randomly extracts a pixel from the first image; and a criterion determination unit which determines whether or not a partial derivative in the pixel extracted by the pixel extraction unit is not smaller than a second threshold, and selecting the pixel as a pixel used for deriving the conversion parameter on a condition that the partial derivative is not smaller than the second threshold.