Abstract: A hidden burglar alarm device for a door or window includes a casing, an alarm mechanism mounted in the casing, a separator detachably inserted into the casing, a magnetic set detachably connecting the separator to the casing, a connector connected with the separator, a ballast fixed on the connector and a flexible wire connected with the ballast. The alarm mechanism includes an alarm member and a trigger connected with the alarm member. The separator selectively abuts against the trigger. The magnetic set includes a metallic object fixed on the separator and a magnet portion fixed on the casing for magnetically attracting the metallic object. The connector has two ends respectively fixed on the separator and the ballast. The flexible wire has two ends respectively fixed on the ballast and a door or window.