A locking device for a semi-fixed leaf cooperating with a mobile leaf to close an opening, the mobile and semi-fixed leaves extending substantially in a plane of the perimeter of the opening when they are both in the closed position, the device comprising: a locking element (10, 100, 10') including a first retaining member (14, 140, 14') able to cooperate with a second retaining member (50, 50') secured to one of the objects from among the semi-fixed leaf and the perimeter of the opening near a meeting point of the two leaves, and a control member (16, 160, 16'), the first retaining member (14, 140, 14') and the control member (16, 160, 16') being able to be movably mounted on the other of said objects such that, when the semi-fixed leaf is in its closed position, the first retaining member (14, 140, 14') and the control member (16, 160, 16') can move substantially parallel to the plane of the perimeter of the opening between a locked configuration, in which the first retaining member (14, 140, 14') is able to cooperate with the second retaining member to lock the semi-fixed leaf, and a released configuration, in which the cooperation of the first and second retaining members can be released; means for urging the locking element (10, 100, 10') toward its released configuration; and a cam (60, 600, 60') intended to be secured to the mobile leaf to cooperate with the control member (16, 160, 16') so as to urge the locking element (10, 100, 10') toward its locked configuration, when the semi-fixed leaf is in its closed position; and a retaining relief (34, 340, 28') able to be secured to said other of said objects and able to cooperate with the second retaining member (50, 50'), when the mobile leaf is open such that the first retaining member (14, 140, 14') is in the released configuration, to form a maintaining hard spot of the second retaining member (50, 50') allowing the semi-fixed leaf to be maintained in the closed position, wherein the first retaining member (14, 140, 14') includes a locking relief which, in the locked configuration, is more pronounced than the retaining relief (34, 340, 28') in the released configuration, such that the retaining relief is formed by the locking relief when the first retaining member is in the released configuration, and such that the second retaining member (50, 50') is elastically movable to catch onto said retaining relief (34, 340, 28') and to be released therefrom when the locking element (10, 100, 10') is in its released configuration. a locking element (10, 100, 10') including a first retaining member (14, 140, 14') able to cooperate with a second retaining member (50, 50') secured to one of the objects from among the semi-fixed leaf and the perimeter of the opening near a meeting point of the two leaves, and a control member (16, 160, 16'), the first retaining member (14, 140, 14') and the control member (16, 160, 16') being able to be movably mounted on the other of said objects such that, when the semi-fixed leaf is in its closed position, the first retaining member (14, 140, 14') and the control member (16, 160, 16') can move substantially parallel to the plane of the perimeter of the opening between a locked configuration, in which the first retaining member (14, 140, 14') is able to cooperate with the second retaining member to lock the semi-fixed leaf, and a released configuration, in which the cooperation of the first and second retaining members can be released; means for urging the locking element (10, 100, 10') toward its released configuration; and a cam (60, 600, 60') intended to be secured to the mobile leaf to cooperate with the control member (16, 160, 16') so as to urge the locking element (10, 100, 10') toward its locked configuration, when the semi-fixed leaf is in its closed position; and a retaining relief (34, 340, 28') able to be secured to said other of said objects and able to cooperate with the second retaining member (50, 50'), when the mobile leaf is open such that the first retaining member (14, 140, 14') is in the released configuration, to form a maintaining hard spot of the second retaining member (50, 50') allowing the semi-fixed leaf to be maintained in the closed position, wherein the first retaining member (14, 140, 14') includes a locking relief which, in the locked configuration, is more pronounced than the retaining relief (34, 340, 28') in the released configuration, such that the retaining relief is formed by the locking relief when the first retaining member is in the released configuration, and such that the second retaining member (50, 50') is elastically movable to catch onto said retaining relief (34, 340, 28') and to be released therefrom when the locking element (10, 100, 10') is in its released configuration.