A laser scanning sensor comprising: a housing in which an opening portion is formed; a cover that is arranged so as to cover the opening portion and adapted to transmit laser light; a laser range finder that is arranged inside the housing, and includes a laser light emitting portion for emitting laser light to outside of the housing through the cover and a laser light receiving portion for receiving laser light and outputting a signal according the amount of the received light, and adapted to perform measurement by acquiring distance information to at least one object through measurement of a period of time taken for reflected light caused by the laser light being reflected by the object to return, and acquiring received light level information of the reflected light; a scanning mechanism portion that is adapted to change a measurement direction performed by the laser range finder; an information acquisition portion that is adapted to form a detection area, as a result of acquisition of the distance information and the received light level information by the laser range finder at each measurement direction while the measurement direction is being changed by the scanning mechanism portion, and that acquires, in time series, the distance information and the received light level information for each measurement direction in the detection area; a storage portion that stores, for each measurement direction, a distance information that corresponds to a farthest side out of the distance information for the measurement direction; a dirt determination portion that determines whether a state in which measurement directions whose distance information corresponds to a near side of a predetermined distance and whose received light level is greater than or equal to a predetermined dirt light level threshold, occupy a predetermined proportion or more of all measurement directions has continued for at least a predetermined period of time; and an alert output control portion that is adapted to output an alert signal according to a result of the determination performed by the dirt determination portion, wherein the predetermined dirt light level threshold is changed based on the distance information stored for each measurement direction in the storage portion, and wherein for each measurement direction, the predetermined dirt light level threshold is increased the closer the distance information stored for the measurement direction in the storage portion corresponds to the near side. a housing in which an opening portion is formed; a cover that is arranged so as to cover the opening portion and adapted to transmit laser light; a laser range finder that is arranged inside the housing, and includes a laser light emitting portion for emitting laser light to outside of the housing through the cover and a laser light receiving portion for receiving laser light and outputting a signal according the amount of the received light, and adapted to perform measurement by acquiring distance information to at least one object through measurement of a period of time taken for reflected light caused by the laser light being reflected by the object to return, and acquiring received light level information of the reflected light; a scanning mechanism portion that is adapted to change a measurement direction performed by the laser range finder; an information acquisition portion that is adapted to form a detection area, as a result of acquisition of the distance information and the received light level information by the laser range finder at each measurement direction while the measurement direction is being changed by the scanning mechanism portion, and that acquires, in time series, the distance information and the received light level information for each measurement direction in the detection area; a storage portion that stores, for each measurement direction, a distance information that corresponds to a farthest side out of the distance information for the measurement direction; a dirt determination portion that determines whether a state in which measurement directions whose distance information corresponds to a near side of a predetermined distance and whose received light level is greater than or equal to a predetermined dirt light level threshold, occupy a predetermined proportion or more of all measurement directions has continued for at least a predetermined period of time; and an alert output control portion that is adapted to output an alert signal according to a result of the determination performed by the dirt determination portion, wherein the predetermined dirt light level threshold is changed based on the distance information stored for each measurement direction in the storage portion, and wherein for each measurement direction, the predetermined dirt light level threshold is increased the closer the distance information stored for the measurement direction in the storage portion corresponds to the near side.