A displacement measuring device, comprising a pattern projecting unit, a pattern image pickup unit capable of relatively displacing with respect to the pattern projecting unit and a control unit, wherein the pattern projecting unit projects a displacement detecting pattern to the pattern image pickup unit, the pattern image pickup unit picks up the displacement detecting pattern as projected, the control unit circulates image of the displacement detecting pattern picked up by the pattern image pickup unit to the pattern projecting unit, updates the displacement detecting pattern projected by the pattern projecting unit to the displacement detecting pattern as circulated, and projects the displacement detecting pattern as updated to the pattern image pickup unit, wherein relative displacement between the pattern projecting unit and the pattern image pickup unit is obtained by dividing a displacement amount of the displacement detecting pattern in the image acquired by circulation by the number of circulations.