-Auxiliary device for guiding a portable rotary drill and for suctioning dust and scraps generated by drilling or countersinking operations performed by the drill, said auxiliary device including: - a first and a second operating unit (11, 12), which are configured to be arranged on opposite sides of an element or of a plurality of elements (C, D) to be drilled, stacked at least partially together, and define respectively a first and a second through channel (13, 16) placed in use coaxially with each other to be engaged by a tip of said drill; - adjustable distance constraint means connecting said first and second operating units (11, 12); - actuating means for selectively actuating said constraint means to adjust the distance between the first and second operating units (11, 12) at a value related to or corresponding to the thickness of the element or of the elements (C, D) to be drilled; and - a channeling system connecting said first and second channel (13, 16) of said first and second operating unit (11, 12) with a suction source to allow the suction of dust and scraps generated by said drill; wherein said constrain means comprise telescopic coupling means (25, 26) between said first and said second operating unit (11, 12); and wherein said telescopic coupling means comprise two telescopically sliding rectilinear elements (25, 26) parallel to said first and second channel (13, 16) and each rigidly connected with a respective one of said first and second operating units (11, 12); characterized in that said actuating means comprise an actuating bar interposed between said first and second operating units (11, 12) so that it is possible to actuate said constraining means from a single side of the auxiliary device itself. - a first and a second operating unit (11, 12), which are configured to be arranged on opposite sides of an element or of a plurality of elements (C, D) to be drilled, stacked at least partially together, and define respectively a first and a second through channel (13, 16) placed in use coaxially with each other to be engaged by a tip of said drill; - adjustable distance constraint means connecting said first and second operating units (11, 12); - actuating means for selectively actuating said constraint means to adjust the distance between the first and second operating units (11, 12) at a value related to or corresponding to the thickness of the element or of the elements (C, D) to be drilled; and - a channeling system connecting said first and second channel (13, 16) of said first and second operating unit (11, 12) with a suction source to allow the suction of dust and scraps generated by said drill; wherein said constrain means comprise telescopic coupling means (25, 26) between said first and said second operating unit (11, 12); and wherein said telescopic coupling means comprise two telescopically sliding rectilinear elements (25, 26) parallel to said first and second channel (13, 16) and each rigidly connected with a respective one of said first and second operating units (11, 12); characterized in that said actuating means comprise an actuating bar interposed between said first and second operating units (11, 12) so that it is possible to actuate said constraining means from a single side of the auxiliary device itself.