Projection optical system and object detection device

An object detection device comprising:

• a light source to emit light;
• an optical element including an incidence plane on which the light from the light source is incident and an emission plane having a plurality of flat planes to emit the light;
• and
• a light-receiving optical system to receive the light emitted from the optical element and reflected by an object, wherein
• when one direction parallel to a light-emitting surface of the light source is defined as a first direction, a direction having a right angle to the first direction which is a direction parallel to the light-emitting surface is defined as a second direction, directions having the right angle to both of the first direction and the second direction are defined as third directions, and an angle range to emit light having predetermined emission intensity or more in a predetermined direction is defined as a divergent angle, the light emitted from the light source has a first divergent angle in the first direction with the third direction as a center and a second divergent angle in the second direction with the third direction as a center, and
• the plurality of flat planes of the optical element includes a first inclination plane inclining in a direction of the light-emitting surface in the first direction.