The invention relates to a locking device at a spagnolet mechanism (1), anchored in a door or window case (39), which mechanism comprises a central adjusting mechanism (2), from which locking rods (7) originate in mutually opposite directions, the free ends of the locking rods (7) being anchored in recesses (41) in a frame (40), a locking bolt (42) being responsible for the locking, which is pivotable around a stationary revolving spindle pin (44) by the locking rods, which locking bolt (42) via a connection link (43) with joints adjacent its ends in a locking house (16) is connected to the associated locking rod. According to the invention the locking bolt (42) is partially inserted into the locking house (16) and is anchored there by way of the revolving spindle pin (44). The locking bolt is connected in an extreme outwardly pivoted position to the connection link (43) via its first joint (45) and is pivoted into the locking house jointly with connection link, which in the locking house is connected to its second joint (46) with the corresponding locking rod end, which jointly with the adjusting mechanism (2) prevent the locking bolt (42) from pivoting back to the extreme position. In this way the unity of the coupling is secured.