An alarm device for an automatic garage door includes a detecting circuit for producing a single negative pulse signal when a detector detects an opening motion of said garage door, an alarm controlling circuit for receiving said single negative pulse signal and producing an alarm signal, a "normal function" detecting circuit connecting to a garage door opening machine for producing a "normal opening" signal when said garage door is normally opened by said garage door opening machine, which is then changed to a "shut off alarm" signal provided to said alarm controlling circuit for preventing the production of said alarm signal, an alarm generating circuit for generating an alarm when said alarm signal is received, and a transformer circuit for providing low voltage for said alarm device. Accordingly, the alarm device for an automatic garage door can provide a warning alarm when said garage door is forced to open or damaged by bandits or burglars. However, as long as the residents operate a normal remote control or in-house switch of said garage door to open or close it, said garage door will be opened and closed as usual (with no alarm).