There is here disclosed a movement error compensating device for a movable unit such as a slider in a numerical control machine tool. A laser beam is emitted from a reference position beam generator installed on a fixed base in the movement direction of the slider. The slider is provided with a beam passing position detector, which detects a deviation on YZ planes of the laser beam. In order to properly detect the irradiation direction of the laser beam, the fixed base is equipped with a movement end beam position detector.