PURPOSE: To collect the dust fallen from stairs automatically by stretching a flexible belt over the full length of the underside of a moving stairs, and providing a winding device in one end of the flexible belt, and providing rewinding device, and a detector of the dust on the flexible belt in the other end of the flexible belt.

CONSTITUTION: A flexible belt 10 is stretched over the full length of the underside of a moving stairs 3, and both ends of the flexible belt 10 are wound around of drums of a winding device 9 and a rewinding device 8. The winding device 9 is provided in a lower mechanic chamber 4. When a predetermined quantity of the dust, which are fallen from clearances of the stairs 3, exists on the flexible belt 10, deflection thereof is detected by a detector 14, and the winding device 9 is driven to collect the dust on the flexible belt 10 in a dust collecting box provided under the winding drum. When the dust on the flexible belt 10 is eliminated after the dust is collected, the flexible belt 10 is rewound to the original position. Consequently, the dust can be collected automatically.