The method comprises the steps of: capturing an output image projected on an actual projection surface; outputting imaging information that includes brightness information having a correlation to a brightness; and correcting a distortion of the output image in relation to the input image according to the imaging information. The distortion correcting step comprises the steps of: providing the image projector with a correction image for using in compensation of the distortion of output image; and determining a peak position movement amount, and also distorting in advance the input image so as to compensate for the distortion. The correction image is configured such that an angle change amount of the peak position according to the peak position movement amount, seen from the image projector is closer to the angle of the slope than an all white pattern image.