PURPOSE: To make it possible to offset additional torque generated by gravity in a broad developing range by compensating the gravity applied on a movable part which performs developing motion with a rotary hinge shaft that is fixed to a fixing part as the center with an elastic tensile material.

CONSTITUTION: When a rib is turned, an elastic tensile member 4 is expanded and contracted, and gravity torque is balanced with the torque due to tension. Here, gravity applied on a movable part which performs developing motion with a rotating hinge shaft that is fixed to a fixing part as the center is compensated with the elastic tensile member. In this way, additional torque generated by the gravity can be offset in a broad developing range.