A system for monitoring an area to be monitored, characterized by comprising: a monitoring apparatus that radiates laser light at predetermined scanning angles (A(n)) and detects an intruding object in the area to be monitored, based on light reflected of the laser light, the area to be monitored being provided by a boundary within an area (R1, R2) composed of a first detection area and a second detection area, the first detection area being set near the monitoring apparatus, the second detection area being set farther than the first detection area when viewing from the monitoring apparatus in directions along the scanning angles (A(n)), the first and second detection areas (R1, R2) being adjacent to each other in the directions along the scanning angles (A(n)); and a reflector provided on the boundary located in the second detection area, the reflector reflecting the laser light radiated from the monitoring apparatus towards the monitoring apparatus in a direction of the radiated light from the monitoring apparatus, wherein the monitoring apparatus includes a radiating unit configured to radiate the laser light towards and scan the area at each of the predetermined scanning angles (A(n)), the laser light radiated from the radiating unit having a spot area at the boundary every one or required ones of the scanning angles, the reflector being provided to be within the spot area; a light receiving unit configured to receive reflected light of the laser light radiated from the radiating unit; a first detecting unit that detects the intruding object at the predetermined scanning angle in the first detection area, wherein the first detecting unit is configured to measure a distance to the object based on an elapsed time from when the laser light is radiated until the reflected light is received by the light receiving unit; and a second detecting unit that detects the intruding object at the predetermined scanning angle in the second detection area, wherein the second detecting unit is configured to compare a light reception amount at a timing at which reflected light is received when the radiated laser light is reflected by the reflector, and a reference light reception amount set in advance to determine whether or not an object is present between the monitoring apparatus and the reflector. a monitoring apparatus that radiates laser light at predetermined scanning angles (A(n)) and detects an intruding object in the area to be monitored, based on light reflected of the laser light, the area to be monitored being provided by a boundary within an area (R1, R2) composed of a first detection area and a second detection area, the first detection area being set near the monitoring apparatus, the second detection area being set farther than the first detection area when viewing from the monitoring apparatus in directions along the scanning angles (A(n)), the first and second detection areas (R1, R2) being adjacent to each other in the directions along the scanning angles (A(n)); and a reflector provided on the boundary located in the second detection area, the reflector reflecting the laser light radiated from the monitoring apparatus towards the monitoring apparatus in a direction of the radiated light from the monitoring apparatus, wherein the monitoring apparatus includes a radiating unit configured to radiate the laser light towards and scan the area at each of the predetermined scanning angles (A(n)), the laser light radiated from the radiating unit having a spot area at the boundary every one or required ones of the scanning angles, the reflector being provided to be within the spot area; a light receiving unit configured to receive reflected light of the laser light radiated from the radiating unit; a first detecting unit that detects the intruding object at the predetermined scanning angle in the first detection area, wherein the first detecting unit is configured to measure a distance to the object based on an elapsed time from when the laser light is radiated until the reflected light is received by the light receiving unit; and a second detecting unit that detects the intruding object at the predetermined scanning angle in the second detection area, wherein the second detecting unit is configured to compare a light reception amount at a timing at which reflected light is received when the radiated laser light is reflected by the reflector, and a reference light reception amount set in advance to determine whether or not an object is present between the monitoring apparatus and the reflector. a light receiving unit configured to receive reflected light of the laser light radiated from the radiating unit; a first detecting unit that detects the intruding object at the predetermined scanning angle in the first detection area, wherein the first detecting unit is configured to measure a distance to the object based on an elapsed time from when the laser light is radiated until the reflected light is received by the light receiving unit; and a second detecting unit that detects the intruding object at the predetermined scanning angle in the second detection area, wherein the second detecting unit is configured to compare a light reception amount at a timing at which reflected light is received when the radiated laser light is reflected by the reflector, and a reference light reception amount set in advance to determine whether or not an object is present between the monitoring apparatus and the reflector.