To detect a foreign matter that attaches to a windshield and to always and correctly perform raindrop detection by dividing a raindrop detection area into a prescribed number of blocks, calculating illuminance change quantity in each block and deciding that a foreign matter attaches when a prescribed number of blocks or more which are equal to and more than a prescribed reference continuously exist.

An image processing part 10 fetches image data from a CCD camera 2 via an image inputting part 9. A raindrop detection area is divided into a prescribed number of blocks and the illuminance of each block is measured and undergone differentiation processing. In the next illuminance change quantity classification processing, the illuminance change level of each block is classified into prescribed stages. In foreign matter detection processing, the level of each block is separately scanned, and identification processing of raindrops and foreign matters is performed by detecting the continuity of blocks that show the illuminance change quantity which exceeds a prescribed reference value. When a foreign matter attaches, a foreign matter indicator 17 is turned on and displayed.