The invention relates to a burglary prevention device for single or double sash, inward-opening windows, wherein a known bolt lock is disposed in the hardware groove of the window sash (6), said lock comprising protruding lugs (7) or pins (8) at the locking point for engaging in the window frame (4) at suitable locking points (3) and thereby locking the lugs (7) or pins (8) to the left or the right or above or below, designed and refined with respect to improved burglary prevention, making a break-in by means of levering out the windows impossible or at least significantly more difficult, such that the hollow chamber between the window frame (4) and the window sash (6) is filled by a security block (1) at least in the areas of the locks, wherein the security block (1) nearly completely eliminates the outer gap between the window frame (4) and the window sash (6), so that a potential attack from the outside of the window on the displaceable parts of the window hardware is prevented.