A document camera has an image processing apparatus, which acquires the contour of an image of an original by means of a Roberts filter. The image processing apparatus detects straight lines as candidates for forming an image of the original from the acquired contour and acquires the shape of a quadrangle of the original. The image processing apparatus determines the projection parameters showing the relationship between the shape of the image of the original and the shape of the actual original from the positions of the corners of the quadrangle and executes an operation of projection/transformation on the image of the original. The document camera outputs the image data of the image to a projector, which projects an image of the original on a screen according to the image data.