PURPOSE: To prevent bounding, reduce time lag and obtain a device in which trouble due to insufficient shunting of a main mirror hardly occurs by braking the main mirror by a brake plate while the main mirror is running from observing position to photographing position.

CONSTITUTION: A mirror operation lever 13 that supports the main mirror is pivotably supported by a shaft together with a driving lever 5, and the lever 5 is energized clockwise by a spring 21, and the lever 13 is energized counterclockwise, that is, in the direction that lowers the main mirror. Further, the lever 5 is energized clockwise by a powerful mirror up spring 14. When a release hook of a camera is depressed, the lever 5 is turned quickly clockwise by energizing force of the spring 14. At this time, speed of rotation of the lever 5 is diminished by friction of a brake plate 15. However, the sliding part of the brake plate 15 is separated from the lever 5 immediately before completion of upward moving of the main mirror and accordingly, the main mirror is pushed up by strong force, and stopping without going up perfectly does not occur. As the speed of upward movement is diminished by the brake plate until that time, bounding is made very small.