PURPOSE: To simplify the device and save the installation and operating costs by incorporating such a constitution that a belt is supported and released according to the width of a rail to guide a supporting device.

CONSTITUTION: In a supporting device 31 sent off from a drum by a wire, the width of pulleys 34, 34a is wider as the spacing between rails 25, 26 and 25a, 26a is greater. Therefore, the tips of movable arms 37, 37a are separate from fixed arms 36, 36a so as to provide a belt 21 with flat condition. When the spacing between the right rails 25, 26 and left rails 25a, 26a becomes smaller, two sets of supporting plates 32, 32a and 33, 33a are moved inward to cause rotation of the movable arms 37, 37a to lead to pinch of the supporting parts 22, 22a of belt 21 with said fixed arms 36, 36a, and now the belt 21 performs transport of particles while bent in approx. U form.