PURPOSE: To eliminate and the need for an optical system which splits and guides part of image projecting light beams to an image sensor and to reduce the size and weight of an optical system by allowing the image sensor to enter the optical path of the image projecting light beams reaching a screen.
CONSTITUTION: A line sensor assembly 40 is equipped with a narrow-width reflecting plate 46 which refracts part of the image projecting light beams striking on the screen 28 by 45° and a line sensor 48 as the image sensor ar ranged in parallel to the reflecting plate 46. A control means 50 makes a focus ing discrimination according to the output signal of the line sensor 48 to move a projection lens 20 to a focusing position as automatic focusing operation, controls a printer 34, moves reflecting mirrors 22, 24, and 30. Then a knob 44 is turned to plate the line sensor assembly 40 at the position where an area including the image of the projecting light is made incident on the line sensor 48, and then part of the projection light is guided to the line sensor 48 by the reflecting plate 46. Consequently, an independent optical system which guides the projecting light beams to the image sensor is not required and the device is reduced in size and weight.