An obstacle detection system for detecting an obstacle inside a level crossing, comprising: a detection unit configured to apply an electromagnetic wave to a reflector installed at the level crossing and receive a reflected wave of the electromagnetic wave; a control unit configured to determine an environmental state inside the level crossing based on the reflected wave; and an output unit configured to output an obstacle presence signal indicating presence of the obstacle or an obstacle absence signal indicating the absence of the obstacle to an outside, wherein: the detection unit is configured to apply the electromagnetic wave to a detection region including a monitor region set inside the level crossing and the reflector so as to scan the detection region in a main scanning direction and a sub scanning direction and receive the reflected wave, the control unit is configured to determine whether or not an obstacle is present in the monitor region based on the reflected wave from the monitor region, the output unit is configured to output the obstacle presence signal when the control unit determines that the obstacle is present, the detection unit has an internal path configured to propagate the electromagnetic wave, the detection unit is configured to allow the electromagnetic wave to enter at a first end of the internal path and receive an emitted wave emitted from a second end of the internal path for each main scan, and the control unit is configured to determine whether or not the detection unit has failed based on the emitted wave. a detection unit configured to apply an electromagnetic wave to a reflector installed at the level crossing and receive a reflected wave of the electromagnetic wave; a control unit configured to determine an environmental state inside the level crossing based on the reflected wave; and an output unit configured to output an obstacle presence signal indicating presence of the obstacle or an obstacle absence signal indicating the absence of the obstacle to an outside, wherein: the detection unit is configured to apply the electromagnetic wave to a detection region including a monitor region set inside the level crossing and the reflector so as to scan the detection region in a main scanning direction and a sub scanning direction and receive the reflected wave, the control unit is configured to determine whether or not an obstacle is present in the monitor region based on the reflected wave from the monitor region, the output unit is configured to output the obstacle presence signal when the control unit determines that the obstacle is present, the detection unit has an internal path configured to propagate the electromagnetic wave, the detection unit is configured to allow the electromagnetic wave to enter at a first end of the internal path and receive an emitted wave emitted from a second end of the internal path for each main scan, and the control unit is configured to determine whether or not the detection unit has failed based on the emitted wave. the detection unit is configured to apply the electromagnetic wave to a detection region including a monitor region set inside the level crossing and the reflector so as to scan the detection region in a main scanning direction and a sub scanning direction and receive the reflected wave, the control unit is configured to determine whether or not an obstacle is present in the monitor region based on the reflected wave from the monitor region, the output unit is configured to output the obstacle presence signal when the control unit determines that the obstacle is present, the detection unit has an internal path configured to propagate the electromagnetic wave, the detection unit is configured to allow the electromagnetic wave to enter at a first end of the internal path and receive an emitted wave emitted from a second end of the internal path for each main scan, and the control unit is configured to determine whether or not the detection unit has failed based on the emitted wave.