A locking device comprises a fixing tube (2) on which an object can be arranged. The fixing tube (2) has a through hole (21) for the tube (3) and a locking sleeve assembly (43) attached to the through hole (21) of the fixing tube (2). The locking sleeve assembly (43) comprises a locking sleeve attachable to the tube (3). The locking sleeve is provided with a fixing member. The locking sleeve assembly (43) further comprises a casing (411) covering the locking sleeve, wherein the casing (411) is provided with a strip opening (412) or slot for the fixing member so that the fixing member protrudes beyond the outer wall of the casing (411), wherein the opening is formed to pivotably guide the fixing member. The locking device may be used with an adjustable leg of an anglers fishing seat box
