PURPOSE: To exactly recognize the number displayed on each number plate by picking up clear image each of three kinds of number plates composed of a background reflection plate, a non-reflection plate and a character reflection plate, in particular, of various kinds of number plates for a vehicle with one camera.

CONSTITUTION: When a vehicle detection sensor 5 detects a vehicle CA, a CCD camera 1 performs the image pickup of the first time. From the image, a number plate LP is extracted and a luminance value is calculated. Based on this calculated luminance value, the diaphragm value of a lens 2 with diaphragm adjusting function is readjusted and the image pickup of the second time is performed. From the image picked up the second time, the reextraction and the recognition of the number plate LP are performed.