A vehicular reflective optical sensor comprising: a control unit configured to emit detection light at prescribed intervals (T1, T2, T3) from a light-emitting unit on an exterior panel of a vehicle towards a detection region set outside the vehicle, to test for a threshold-exceeded state by comparing an amount of light in a light reflection received from the detection region by a light-receiving unit to a detection threshold (Th on) and detecting the threshold-exceeded state when the amount of light is greater than or equal to the detection threshold (Th on), to execute a testing step of the threshold-exceeded state by determining whether the detection threshold (Th on) is exceeded continuously for a prescribed number of light reflections to confirm whether the detected threshold-exceeded state is attributed to the presence of a detection target in the detection region, to output a detection-confirmation signal if it is detected that a detection target has entered the detection region, wherein the control unit uses, as the detection threshold (Th on), a sum of a predetermined fixed value (Th onfix) and an adjustment value (Th cal) and wherein the adjustment value is calculated by obtaining an average value of the amounts of light (P 1 to P 10) in a prescribed number of light reflections preceding the light reflection on which the comparison is being performed. a control unit configured to emit detection light at prescribed intervals (T1, T2, T3) from a light-emitting unit on an exterior panel of a vehicle towards a detection region set outside the vehicle, to test for a threshold-exceeded state by comparing an amount of light in a light reflection received from the detection region by a light-receiving unit to a detection threshold (Th on) and detecting the threshold-exceeded state when the amount of light is greater than or equal to the detection threshold (Th on), to execute a testing step of the threshold-exceeded state by determining whether the detection threshold (Th on) is exceeded continuously for a prescribed number of light reflections to confirm whether the detected threshold-exceeded state is attributed to the presence of a detection target in the detection region, to output a detection-confirmation signal if it is detected that a detection target has entered the detection region, wherein the control unit uses, as the detection threshold (Th on), a sum of a predetermined fixed value (Th onfix) and an adjustment value (Th cal) and wherein the adjustment value is calculated by obtaining an average value of the amounts of light (P 1 to P 10) in a prescribed number of light reflections preceding the light reflection on which the comparison is being performed. to emit detection light at prescribed intervals (T1, T2, T3) from a light-emitting unit on an exterior panel of a vehicle towards a detection region set outside the vehicle, to test for a threshold-exceeded state by comparing an amount of light in a light reflection received from the detection region by a light-receiving unit to a detection threshold (Th on) and detecting the threshold-exceeded state when the amount of light is greater than or equal to the detection threshold (Th on), to execute a testing step of the threshold-exceeded state by determining whether the detection threshold (Th on) is exceeded continuously for a prescribed number of light reflections to confirm whether the detected threshold-exceeded state is attributed to the presence of a detection target in the detection region, to output a detection-confirmation signal if it is detected that a detection target has entered the detection region, wherein the control unit uses, as the detection threshold (Th on), a sum of a predetermined fixed value (Th onfix) and an adjustment value (Th cal) and wherein the adjustment value is calculated by obtaining an average value of the amounts of light (P 1 to P 10) in a prescribed number of light reflections preceding the light reflection on which the comparison is being performed.