1. A pavement surface inspecting device used in a civil engineering field for inspecting a road surface, comprising: a pair of light modules being sequentially activated and providing flash lights incident to a pavement surface from two different incident angles, to cause an unevenness distributed with an irregular region on the pavement surface to be exposed on the pavement surface in a form of a light shadow, in which the incident angles are independent of the unevenness; an image sensing module being activated to acquire images of the pavement surface while one of the light modules are activated, which images contain the light shadow for the unevenness shown on the pavement surface; and a computing module electrically connected with the image sensing module, receiving the acquired images, transforming the respective acquired images into a digital grayscale image consisting of a plurality of grayscale pixels, each of which grayscale pixels have a grayscale value, subtracting pixel by pixel one of the transformed digital grayscale images from the other to generate a single processed image consisting of a plurality of processed pixels, each of which processed pixels have a discrepant value and among which processed pixels the multiple pixels have the discrepant value greater than a threshold are identified as uneven pixels, and determining the irregular region on the pavement surface to which the uneven pixels are corresponding as an uneven region on the pavement surface.