Abstract: A burglarproof device for a sliding leaf including at least a locking organ capable of being protruding with respect to the front edge of the sliding leaf, so as to cooperate with a keeper inserted against the corresponding stile of the sash-frame. This burglarproof device includes a burglar-safe case defined by an inner wall and an outer wall extending perpendicularly to a bottom provided with an elongated slot through which passes the locking organ; and a keeper formed by a cap including an opening for receiving the locking organ, this cap being capable of covering a bearing capable of being inserted in the fillister of the sash-frame. This cap has a shape without undercut impeding a tool from being hooked in during an attempt of burglary.