PURPOSE: To attain an optical filter where a desired filter characteristic can be used freely selectively, by providing continuously an optical thin film, whose thickness is changed gradually, in the lengthwise direction, on a long-sized transparent substrate such as a transparent plastic substrate.

CONSTITUTION: An optical thin film (for example, an Ni-Cr alloy film) 2 is formed on a long-sized substrate 1 consisting of a transparent plastic (for example, polyester) while changing the thickness of the film 2 in the lengthwise direction, thus forming a product film (neutral density filter) 3. This product film 3 is stretched between a pair of reels 11 and 11 through driving rolls 12 and 12 so that the product film 3 can be moved in the lengthwise direction, and a prescribed part of the film 3 is allowed to face an optical path 13, thereby using this film freely as the filter having a desired characteristic.