1. A method of determining normality of an imaging direction executed by a computer, the method comprising: acquiring from an imaging device attached to a vehicle, image data in each of a plurality of frames in each of which a reference direction is defined, when a speed of the vehicle is equal to or greater than a first threshold and an acceleration of the vehicle is less than a second threshold; calculating, for each area of the image data in the reference direction, an index related to an amount of a difference between the frames of the image data; and evaluating an attachment state of the imaging device on a basis of distribution of the indices related to the amount of the difference in the reference direction wherein the index related to the amount of the difference is the amount of the difference of luminance between the frames of the image data, and the evaluating creates, for a position of each of a plurality of pixels in the reference direction, distribution of integrated values of luminance of the pixels arrayed in a direction intersecting the reference direction, determines the attachment state of the imaging device is abnormal when an upper position of the distribution is greater than a lower position of the distribution and determines the attachment state of the imaging device is normal when the upper position of the distribution is not greater than the lower position of the distribution.