In an ambient environment determination apparatus, an imager obtains a picture capturing an area ahead of a vehicle, and a street lamp is detected for each detection frame unit of the picture. Then, an urban area determination process is performed that determines whether or not the ambient environment of the vehicle is an urban area based on both of a street lamp detection result of a current detection frame unit and a street lamp detection result of a past detection frame unit of the picture. Further, in a period after the vehicle turns right or left, determination responsiveness with regard to determination whether or the ambient environment is a urban area or a non-urban area is enhanced than that in a period other than the period after the right or left turn.