PURPOSE: To improve visibility through setting of an image forming position to a position wherein each driver is easy to confirm visually by providing an optical path length varying means through the working of which the length of an optical path for information light formed between a display source and a reflection member is variable.

CONSTITUTION: A reduction gear 13 is rotated in a direction shown by an arrow mark F with the aid of a step motor 11 and a second holder 10 having a spur gear 102 geared with the reduction gear 13 is pushed up along a shaft 8. A half mirror 7 formed integrally with the second holder 10 is moved in the direction of an arrow mark U (upward) in a state to keep a given inclination angle. In this case, a first holder 9 is functioned as an auxiliary member through operation of which the given angle of the half mirror 7 is reliably kept. By moving the half mirror 7 to a topmost position, the distance of an optical path shown by an arrow mark B is shortened and information of a display unit 4 is visually confirmed as a virtual image 11, formed in a near place, by a driver.