An edge detecting device for detecting a position of an edge of an object such as a sheet, comprising: a device main body, reflective means for reflecting an inspection light, wherein said reflective means is a sheet cover for covering said object, and wherein said sheet cover is connected to an upper face of the device main body in an openable and closeable manner, measuring means adapted for emitting said inspection light to a linear inspecting area including said object, and measuring a first light quantity of reflected light of said inspection light reflected from said inspecting area in a semi-opened state, in which said sheet cover is not completely closed on said inspecting area, and a second light quantity of reflected light of said inspection light reflected from said inspecting area in a closed state, in which said sheet cover is completely closed on the inspecting area, calculation means adapted for - calculating a difference between said first light quantity and said second light quantity measured by said measuring means for each of the pixels in all or in a part of a line of pixels in said linear inspecting area, - for each of said pixels, considering the pixel as an objective dot bordered on one side and on the other side by a first predetermined number of adjacent pixels, and - calculating a first total of differences of reflected lights from said adjacent pixels on said one side by totalizing differences between said first and said second light quantity reflected from each of the said adjacent pixels on said one side, and - calculating a second total of differences of reflected lights from said adjacent pixels on said other side by totalizing differences between said first and said second light quantity reflected from each of the said adjacent pixels on said other side; and judging means adapted for judging one of said objective dots with the largest variation between the first total difference and the second total difference to be a position of said edge, wherein the variation of an objective dot is calculated as the absolute value of the difference between the first and second total difference at said objective dot. a device main body, reflective means for reflecting an inspection light, wherein said reflective means is a sheet cover for covering said object, and wherein said sheet cover is connected to an upper face of the device main body in an openable and closeable manner, measuring means adapted for emitting said inspection light to a linear inspecting area including said object, and measuring a first light quantity of reflected light of said inspection light reflected from said inspecting area in a semi-opened state, in which said sheet cover is not completely closed on said inspecting area, and a second light quantity of reflected light of said inspection light reflected from said inspecting area in a closed state, in which said sheet cover is completely closed on the inspecting area, calculation means adapted for - calculating a difference between said first light quantity and said second light quantity measured by said measuring means for each of the pixels in all or in a part of a line of pixels in said linear inspecting area, - for each of said pixels, considering the pixel as an objective dot bordered on one side and on the other side by a first predetermined number of adjacent pixels, and - calculating a first total of differences of reflected lights from said adjacent pixels on said one side by totalizing differences between said first and said second light quantity reflected from each of the said adjacent pixels on said one side, and - calculating a second total of differences of reflected lights from said adjacent pixels on said other side by totalizing differences between said first and said second light quantity reflected from each of the said adjacent pixels on said other side; and - calculating a difference between said first light quantity and said second light quantity measured by said measuring means for each of the pixels in all or in a part of a line of pixels in said linear inspecting area, - for each of said pixels, considering the pixel as an objective dot bordered on one side and on the other side by a first predetermined number of adjacent pixels, and - calculating a first total of differences of reflected lights from said adjacent pixels on said one side by totalizing differences between said first and said second light quantity reflected from each of the said adjacent pixels on said one side, and - calculating a second total of differences of reflected lights from said adjacent pixels on said other side by totalizing differences between said first and said second light quantity reflected from each of the said adjacent pixels on said other side; and judging means adapted for judging one of said objective dots with the largest variation between the first total difference and the second total difference to be a position of said edge, wherein the variation of an objective dot is calculated as the absolute value of the difference between the first and second total difference at said objective dot.