An apparatus and a method are proposed for measuring displacement, surface profile and roughness of a moving object or an inner radius of a hollow cylinder. The apparatus includes a light emitting unit, a light dispersing unit for receiving light from the light emitting unit and focusing rays with different wavelengths into different focal points with different intervals, and a wavelength measuring unit for measuring wavelengths of the rays. When the moving object is moving within a dispersing range of the focal points, the rays with different wavelengths are reflected or scattered, and the displacement of the moving object is learned from variation of the wavelengths being measured by the wavelength measuring unit. Given that a reflecting component is disposed in the centre of an inner circle of the hollow cylinder, the inner radius of the hollow cylinder can be measured by the principle of the apparatus for measuring displacement.
