To provide a burglar alarm to be usable where a distance is short between an auxiliary lock and one of window/door frame sides and to prevent damage of a sensor.

This easily attachable burglar alarm sensor 1 is provided with a sensor 2, a battery, an alarm means and a control circuit and is attached to a side 53 of one window/door frame 51 of double sliding windows/doors 5. In an alarming mode, the sensor part 22 of the above sensor 2 rests on the top surface 31 of the auxiliary lock 3 attached to the other window/door frame 52 and the alarm means is sounded when the sensor part 22 is dropped from the top surface 31 of the auxiliary lock 3.