To quickly and easily re-form a monitor surface for monitoring an intrusion of an object into a dangerous area when a second conveyer 14 of a ply molding machine is deformed (rotationally turned).

A one-side monitoring sensor body 52, a one-side support body 38, and a one-side base 37 are provided on a second conveyer 14 of a ply molding machine, so that, when the second conveyer 14 is displaced (rotationally turned) and a dangerous area is changed, the one-side monitoring sensor body 52 and the like are followingly displaced in a manner integrated with the second conveyer 14, to dispense with work for moving them by a worker. Furthermore, the one-side and other-side support bodies 38 and 48 are rotatably supported by the one-side and other-side bases 37 and 45. Consequently, when the dangerous area is changed, just the one-side and other-side support bodies 38 and 48 are rotated and the rotating positions are adjusted, so that the one-side and the other-side monitoring sensor bodies 52 and 53 are easily and quickly made to face each other so as to re-form the monitoring surface 55.