1. A head mounted display comprising: a display unit which is configured to project an image light having intensity corresponding to image information on an eye of a user thus allowing the user to visually recognize an image corresponding to the image light while allowing an external light to pass therethrough; an imaging unit which is configured to image a predetermined range including at least a range where the image is displayed out of a field-of-view range of the user; an image forming unit which is configured to form a background cancelling image based on an image imaged by the imaging unit, wherein when the background cancelling image is superposed on a background image derived from the external light and visually recognized by the user, the background cancelling image superposed on the background image constitutes an image having reference background brightnesses with a predetermined constant brightnesses within a predetermined frame period for respective three primary colors and thereby cancels the background image, and wherein the background cancelling image is formed by the following steps: i) determining a brightness of the background image for each color, ii) calculating a difference in a brightness between the background image and a reference background brightness for each color and iii) forming the background cancelling image having a respective brightness for each color based on the difference in the brightness between the background image and the reference background brightness for each color; an image processing unit which is configured to combine the background cancelling image with an original image corresponding to original image information by aligning the background cancelling image and the original image with each other in terms of display position and size; and a display control unit which is configured to perform a control by which an image to which the background cancelling image is combined is displayed by the display unit.