1. An image composition system for compositing a real image in a line-of-sight direction of a user with another image, comprising: a display unit which is wearable on a head of the user, and displays a composite image; a position sensor for detecting the line-of-sight direction of the user, and outputting line-of-sight information; a determination unit for determining a display region where the other image is to be displayed, in accordance with the line-of-sight information; and a composition unit for compositing the other image on the determined display region, wherein the other image is used to display information that helps operations of the user.