A device for measuring intensity of a transmitted light, includes a first measuring unit that measures a first intensity of a light transmitted through a medium on a conveying path in its thickness direction and outputs a first measured value; a second measuring unit that is arranged adjacent to the first measuring unit, measures a second intensity of the light transmitted through the medium in the thickness direction, and outputs a second measured value; and an operating unit that obtains a true measured value from the first measured value and the second measured value.