1. A rain sensor for a vehicle, comprising: a camera provided in the vehicle configured with a long focal length set to take an image of a scene outside of the vehicle through a windshield of the vehicle while blurring a substance on the windshield; and a processor unit configured to detect rain based on a variation degree of intensities of pixels, which are included in the image, from an average intensity of the pixels, wherein the processor unit is configured to divide an image taken by the camera into sub-images according to a plurality of sub-areas defined in an area of the image; calculate, with respect to each of the plurality of sub-areas, a moment-to-moment transition of a variation degree of intensities of pixels, which are included in the each of the plurality of sub-areas, from an average intensity of the pixels included in the each of the plurality of sub-areas, and detect rain based on the moment-to-moment transition.