To provide a burglary preventing device whose cassette housing part is formed separately from an alarm issuing part and capable of reusing one part even when another part is damaged.

An openable/closable casing 1 is constituted by pivotally supporting and connecting a casing main body 2 with a cover 3 at a pivotally supporting part 11. A cassette housing chamber 13 to be an object of burglary prevention is formed inside the casing main body 2 and an alarm unit 4 to issue an alarm by protruding an equipped alarm issuing switch piece 42 to an alarm issuing position is mounted attachably/detachably at the side of the cassette housing chamber 13. The cover 3 holds the alarm issuing switch piece 42 of the alarm unit 4 at a depressed position to set an alarm at an action stand-by state in a closed state against the casing main body 2.