PURPOSE: To obtain a dust protection device of a directly acting mechanism which makes the efficient driving transmission to a movable part and the dust protection and drip protection of a closed support mechanism compatible properly with a simple structure.

CONSTITUTION: A seal part 4A arranged at the aperture 3 peripheral edge at the opposite side in the directly acting direction of a movable part 5 is composed of an upper side seal member 21 arranged at the aperture outside and a lower side seal member 22 arranged at the aperture inside and a steel belt 6 is interposed from both the upper and lower sides by the tip 21A of the upper side seal member 21 and the tip 22A of the lower side seal member 22. Also, a seal part 4B at the opposing side is interposing the steel belt 6 with the tip 23A of the upper side seal member 23 and the tip 24A of the lower side seal member 24 similarly. These seal parts 4A, 4B press both the front and back faces of the steel belt 6 to peel off the dust, liquid drip, etc., sticked to the surface especially by the sliding with the steel belt 6.