To prevent the vibration of a reflection mirror to prevent image shake by supporting ends at both sides in a longitudinal direction, at the side of a reflection surface of the reflection mirror by plural supporting parts.

A third mirror (reflecting mirror) 22 is supported by projecting parts 26 to 28 at three positions. This device is provided with a front opening part 24 and a rear opening part 25 which are opposite to each other at the front and the rear sides of a second mirror stand frame body 23, so that the reflection mirror 22 is inserted into the openings. In addition, the projecting part 26 the tip of which is butted against the reflection mirror 22 is provided at almost the center of mirror width to project from the second mirror stand frame body 23 to the front opening 24. Furthermore, the projecting parts 27, 28 with the same projection quantity and whose tips are butted against the reflecting mirror 22 are provided at almost both ends of the mirror width to project from the second mirror stand frame body 23 to the rear opening 25. The reflecting mirror 22 is pressurized from the back by left sprints 29, 30, and the reflection surface 22a is butted against the projecting parts 26 to 28 and positioned.