A device including a radar and a camera for detecting an object located in front of an automobile is mounted on the automobile. The radar detects a distance from the automobile to the front object, and the camera takes an image of the object. When the object such as a preceding vehicle moves out of a region detectable by the radar while remaining in a region covered by the camera, a present distance to the object is calculated based on the distance previously detected by the radar and memorized in a memory and a present object image taken by the camera. More particularly, the present distance is calculated by multiplying the memorized distance by a ratio of an object size in the memorized image to an object size in the present image.