An overlay measurement device measures an error between a first overlay mark and a second overlay mark respectively formed on different layers formed on a wafer. The device is configured to detect a height of the first overlay mark based on a change in the signal of the first detector according to a change in the relative position of the objective lens with respect to the wafer in the optical axis direction and detect a height of the second overlay mark based on a change in the signal of the second detector according to a change in the relative position of the objective lens with respect to the wafer in the optical axis direction.