A bearing in connection with a telescopically extensible boom (1) comprising an outer arm (5) with a front end and an inner arm (6) arranged in this front end that can be displaced axially inside the outer arm (5). The outer and inner arms consist of square tubes. Flat surfaces of the inner arm (6) run along corresponding flat surfaces of the outer arm (5) and the boom (1) has four longitudinal sides. The bearing comprises a bearing box (10) designed as a casing with four walls (10a - 1Od), which together form a square tube structure. The bearing box (10) comprises a number of sliding bearings (l la - 1 Id, 12a - 12d) and is inserted into the front end of the outer arm (5). The sliding bearings (Ha - 1 Id, 12a - 12d) are arranged in the walls (10a - 1Od) of the bearing box (10) and are applied against the outer (5) and the inner (6) arms' corresponding flat surfaces that run along each other. In connection with at least one wall (10a - 10c) of the bearing box, axially arranged adjustment devices (15, 20, 17a - 17c) are arranged for adjusting the play between at least one sliding bearing (1 Ia - 1 Ic, 12a - 12c) and the applied surfaces of the inner arm (6) and the outer arm (5).