This invention provides a coaxial regressive reflection type photoelectronic sensor capable of enhancing detection accuracy. A sensor head includes a light emitting diode for emitting a detection light; a lens for converting the detection light to a parallel light and projecting the parallel light to an exterior as an external detection light; a half mirror for separating an optical path of the detection light and an optical path of a return light from the regressive reflection plate; and a photodiode for receiving the return light from the half mirror. The lens includes a convex surface formed to convert a signal light to a parallel light, and a plane tilted from a perpendicular direction with respect to the parallel light or an axis connecting a light emitting diode chip and a center portion of the convex surface. The reflected light of an external signal light at the plane passes a path different from that of the return light so as to be avoided from entering the photodiode chip.