The purpose of this invention is to provide a vehicular reflective optical sensor that has improved control reliability. Said vehicular reflective optical sensor is provided with a control unit (6) that: emits detection light (3) at prescribed intervals from a light-emitting unit (2) on an exterior panel of a vehicle towards a detection region (1) set outside said vehicle; tests for a threshold-exceeded state in which the amount of light in a light reflection (4) received from the detection region (1) by a light-receiving unit (5) exceeds a prescribed detection threshold; and, if it is detected that a detection target has entered the detection region (1), outputs a detection-confirmation signal. The control unit (6) uses, as the aforementioned detection threshold, the sum of a predetermined fixed value and an adjustment value comprising a summary statistic for the amounts of light in an appropriate number of light reflections (4) preceding the light reflection (4) on which the comparison is being performed.