In a plasterboard panel (1), an inspection opening (2) and a related closure element (3), comprising a first plasterboard plate (30) having dimensions substantially corresponding to those of the inspection opening (2) and thickness substantially equal to the thickness of the panel (1), are flared upwards and their respective lower faces (21, 31) have exactly the same shape and dimensions. The inspection opening (2) has segments (22) of inner peripheral edge (23) inclined by a predetermined minimum angle ( alpha ) relative to the perpendicular to the panel (1), for the abutment of corresponding, counter-shaped portions (32) of outer peripheral edge (33) of the closure element (3) inclined by the same angle ( alpha ), in order stably to support the element (3) in the position of insertion in the inspection opening (2) with its own lower face (31) substantially coplanar to a lower face (10) of the panel (1). Preferably, the segments (22) of the inner peripheral edge (23) are in mutual contact and together involve the whole inner peripheral edge (23), the portions (32) of the outer peripheral edge (33) being, correspondingly, also in mutual contact and together involving the whole outer peripheral edge (33) of the closure element (3).