In an ink jet printer using an ink cartridge having an ink tank for storing ink therein and a recording head for ejecting the ink stored in the ink tank as ink drops and forming an image on a recording paper sheet, the detection accuracy of a displacement detection sensor for detecting a displacement of printing position is improved while reducing manufacturing costs of the displacement detection sensor. The displacement detection sensor is comprised of a light emitting device for emitting diffused light, a first lens for focusing the light emitted from the light emitting device on a recording paper sheet, a second lens for focusing the light reflected by the recording paper sheet and a light receiving device for receiving the light focused by the second lens. By focusing the diffused light emitted from the light emitting device by the first lens, the diameter of a beam spot projected on the recording paper sheet is made smaller than a longitudinal dimension and a width dimension of each component forming a displacement detection pattern.