An object detector, comprising: a projection optical system (11, 21), and a light-receptive optical system; the projection optical system comprising: a light source, and an optical element (L11, L21), wherein light emitted from the light source is incident on the optical element (L11, L21), the optical element being configured to change an intensity distribution of a light beam emitted from the light source in a first direction, the optical element comprising: a concave incidence surface (S11, S21) on which the light beam is incident; and an exit surface (S12, S22) which is composed of two planes that are joined together to form a ridge line orthogonal to the first direction, thereby forming a convex surface in an exiting direction of the light beam, wherein the optical element (L11, L21) has a constant cross-sectional profile along the ridge line, wherein the projection optical system is configured to irradiate an object to be detected with the light beam and the light-receptive optical system is configured to receive light reflected off the object. a projection optical system (11, 21), and a light-receptive optical system; the projection optical system comprising: a light source, and an optical element (L11, L21), wherein light emitted from the light source is incident on the optical element (L11, L21), the optical element being configured to change an intensity distribution of a light beam emitted from the light source in a first direction, the optical element comprising: a concave incidence surface (S11, S21) on which the light beam is incident; and an exit surface (S12, S22) which is composed of two planes that are joined together to form a ridge line orthogonal to the first direction, thereby forming a convex surface in an exiting direction of the light beam, wherein the optical element (L11, L21) has a constant cross-sectional profile along the ridge line, wherein the projection optical system is configured to irradiate an object to be detected with the light beam and the light-receptive optical system is configured to receive light reflected off the object. a light source, and an optical element (L11, L21), wherein light emitted from the light source is incident on the optical element (L11, L21), the optical element being configured to change an intensity distribution of a light beam emitted from the light source in a first direction, the optical element comprising: a concave incidence surface (S11, S21) on which the light beam is incident; and an exit surface (S12, S22) which is composed of two planes that are joined together to form a ridge line orthogonal to the first direction, thereby forming a convex surface in an exiting direction of the light beam, wherein the optical element (L11, L21) has a constant cross-sectional profile along the ridge line, wherein the projection optical system is configured to irradiate an object to be detected with the light beam and the light-receptive optical system is configured to receive light reflected off the object. a concave incidence surface (S11, S21) on which the light beam is incident; and an exit surface (S12, S22) which is composed of two planes that are joined together to form a ridge line orthogonal to the first direction, thereby forming a convex surface in an exiting direction of the light beam, wherein the optical element (L11, L21) has a constant cross-sectional profile along the ridge line, wherein the projection optical system is configured to irradiate an object to be detected with the light beam and the light-receptive optical system is configured to receive light reflected off the object.