A laser scan sensor, comprising: a laser range finder configured to measure a distance to an object based on a time after a laser beam emits before a reflected light returns from the object present in a direction of the emission; a scanning mechanism unit configured to change a measurement direction of the laser range finder; a distance-information acquiring unit configured to periodically perform measurement using the laser range finder while changing a measurement direction using the scanning mechanism unit, so as to form a detection area and acquire distance information for each measurement direction within the detection area in chronological order; an installation-state storage unit configured to store installation state information of the laser range finder; a detection area-information storage unit configured to store distance information to be a reference for each measurement direction as detection-area information; a human-body determining unit configured to: identify objects that have invaded or moved by comparison of the distance information acquired by the distance information acquiring unit with the detection area information; extract a portion that possibly corresponds to a human body in an object that has invaded or moved; determine a movement state of each of the extracted portions in chronological order, excluding the extracted portions where a movement distance within a predetermined time is within a predetermined distance; and determine whether or not a rest of the respective extracted portions is a human body based on said movement state a detection area-information updater configured to update the detection-area information for each measurement direction under a predetermined condition; and an alert-signal output unit configured to output an alert signal in a case where a human body is determined to be present by the human-body determining unit, wherein the detection area-information updater gradually updates the detection-area information of the measurement direction at a predetermined proportion where said predetermined condition is a case where a height calculated from the distance information for each measurement direction based on the installation state information is equal to or less than a predetermined height. a laser range finder configured to measure a distance to an object based on a time after a laser beam emits before a reflected light returns from the object present in a direction of the emission; a scanning mechanism unit configured to change a measurement direction of the laser range finder; a distance-information acquiring unit configured to periodically perform measurement using the laser range finder while changing a measurement direction using the scanning mechanism unit, so as to form a detection area and acquire distance information for each measurement direction within the detection area in chronological order; an installation-state storage unit configured to store installation state information of the laser range finder; a detection area-information storage unit configured to store distance information to be a reference for each measurement direction as detection-area information; a human-body determining unit configured to: identify objects that have invaded or moved by comparison of the distance information acquired by the distance information acquiring unit with the detection area information; extract a portion that possibly corresponds to a human body in an object that has invaded or moved; determine a movement state of each of the extracted portions in chronological order, excluding the extracted portions where a movement distance within a predetermined time is within a predetermined distance; and determine whether or not a rest of the respective extracted portions is a human body based on said movement state identify objects that have invaded or moved by comparison of the distance information acquired by the distance information acquiring unit with the detection area information; extract a portion that possibly corresponds to a human body in an object that has invaded or moved; determine a movement state of each of the extracted portions in chronological order, excluding the extracted portions where a movement distance within a predetermined time is within a predetermined distance; and determine whether or not a rest of the respective extracted portions is a human body based on said movement state a detection area-information updater configured to update the detection-area information for each measurement direction under a predetermined condition; and an alert-signal output unit configured to output an alert signal in a case where a human body is determined to be present by the human-body determining unit, wherein the detection area-information updater gradually updates the detection-area information of the measurement direction at a predetermined proportion where said predetermined condition is a case where a height calculated from the distance information for each measurement direction based on the installation state information is equal to or less than a predetermined height.