An anti-theft apparatus includes a bracket-shaped auxiliary member which is displaceably guided longitudinally along a narrow side of a casing and has ends which are bent inwardly in order to clamp a box-like article. When the bracket is in its slid out position relative to the casing, its outer end can be bent in an elastic manner to enable removal or insertion of the article, and the inner end of the bracket has a locking lug which also projects inwardly to snap into a locking slot at the narrow side of the casing opposite of the slide-in opening when the bracket is completely inserted into the casing. The locking lug can be released out of this position only by use of a special tool.