A compact fall detection sensor includes a case. A magnet is housed in the case. A reed switch with a pair of reeds are integrally positioned around the central axis of the case at the bottom of the case. In an upright state, the magnet is gravitationally positioned in the longitudinal direction of the reed switch around the central axis of the curved surface of the case, each magnetic pole magnetizing the corresponding reed of the reed switch, allowing one of the reeds to be magnetized to N pole and the other reed to S pole to turn on the switch. When inclined from the upright state, the magnet slides along the curved surface of the case and deviates from the central axis, to position only one of the magnetic poles of the magnet around the central axis to turn off the reed switch.