PURPOSE: To improve workability and safety by arranging a back rest to a seat plate, pivotally supporting a safety frame, with which a worker is surrounded, to the back rest such that it is revolvable at the overhead of a worker, and removably mounting the revolving end side to a fixing part on the opposite side to the back rest.

CONSTITUTION: A seat plate 6 is arranged to scaffolding 17 at the tip of a crane boom 2 such that an angle is regulatable according to the angle of elevation of the crane boom 2. A back rest 41 is arranged to the rear end part of the seat plate 6, a safety frame 32 is disposed to the upper ends of back rest rods 35A and 35B, of which the back rest 41 consists, through a safety frame support member 36 and the base end part thereof is revolvable around the safety frame support member 36 and at the overhead of a worker on the scaffolding 17. The revolving end side forms a fit-in part 37, which is mountable on and demountable from safety frame fixing part 31 arranged on a front support body 4 and revolvable between a position where the head of a worker is covered therewith and a position where the head of the worker is released from covering.