PURPOSE: To provide an outdoor laser alarm in which target detectable distance is increased as long as possible while preventing erroneous alarm due to outdoor environmental conditions.
CONSTITUTION: A pulse laser beam 6 is emitted from a laser driver 2 and projected through an optical transmitting section 7 under control of a controller 1a and reflected light enters through a detector 10 and an amplifier 12 into a comparator 13a. A plurality of reference voltages are prepared for the comparator 13a by a reference voltage generator 14. In prestage for beginning laser detection, noise level due to external environmental conditions such as mist is detected and a reference voltage slightly higher than the noise level is selected. This constitution ensures maximum detectable distance while preventing erroneous alarm due to external environment.