A paper thickness detecting device that detects a paper thickness in accordance with a detection signal of a displacement sensor, includes: a pair of paper thickness detecting rollers including a first roller and a second roller that pinch a paper conveyed via a pair of conveyance rollers; and the displacement sensor that detects a displacement amount of the pair of paper thickness detecting rollers at the time when the paper passes through the pair of paper thickness detecting rollers, wherein the paper thickness is detected on the basis of the displacement amount of the pair of paper thickness detecting rollers that follows to rotate with respect to the paper conveyed via the pair of conveyance rollers.