PURPOSE: To improve the external field recognition rate when the outside of the vehicle is monitored by setting the focal length at the time of image pickup operation to a proper value.

CONSTITUTION: According to the speed of the vehicle 1 detected by a vehicle speed sensor 5, proper focal length is set by a focal length setting means 15 and outputted to an image pickup means 10 and an image processing means 20, and then the zoom mechanism of the image pickup means 10 is driven to pick up an image outside the vehicle with the set focal length. Then the image processing means 20 processes the image picked up on the basis of the focal length at the time of the pickup operation to obtain external field information. Consequently, the focal length is made smaller in a slow-speed range to secure a wider visual field, and consequently the external field state can accurately be grasped; and the focal length is made larger in a high speed range to monitor a farther scene, and an obstacle is detected at a longer distance to improve travel safety.