To accurately measure the distance from one's own vehicle to an obstacle ahead, such as an inter-vehicle distance, based on an image taken with a monocular camera, while minimizing the influence of disturbance and the like if the lateral width of an obstacle ahead of the one's own vehicle, such as the lateral width of a vehicle ahead, is unknown.

The monocular camera 2 is mounted in the one's own vehicle 1 and an image of an area in front of the one's own vehicle is processed by the camera 2 so that a position along the height of the reference part of the obstacle in front of the one's own vehicle, such as a vehicle ahead, on the image taken, is detected as an observation position. A nonlinear-state space model with the input of the observation position is subjected to filtering by an adaptive filter, whereby the distance from the one's own vehicle 1 to the obstacle is estimated and measured on the basis of the observation position and the imaging conditions of the monocular camera 2.