PURPOSE: To achieve work safety when an eccentric position of an offset boom draws the arm to the maximum by limiting drawing of the arm before a work attachment interferes with an operator cab if the attachment is in such a range as to interfere with the cab.

CONSTITUTION: A front work room 30 comprises an offset boom 31 having a lower boom 3, an upper boom 4, a cylinder stay 5, and a work attachment 7 at the forward end of an arm 6 installed over the cylinder stay 5. In the case where the attachment 7 is in such a range as to interfere when an eccentric position of the offset boom 31 draws the arm 6 to the maximum, a safety means is provided to be applied to the arm 6 before occurrence of interference for limiting further drawing of the arm 6. A bucket 7 can thus be prevented from being collided to a cab 2, thereby the safety of work can be secured.