Alarm device for a vehicle

A motion sensor (8) is provided in back of the instrument panel (3) that transmits radio waves continuously or periodically and receives the radio waves reflected in the vehicle compartment. Also, there is provided a burglar horn (13) that executes specified alarming when a specified change in the reflected radio wave received by the motion sensor (8) occurs. The motion sensor (8) transmits the radio waves toward the instrument panel (3). Thereby, a stable burglar alarm operation can be performed.