A linear scattering inverse method based in the Kirchhoff approximation is formulated to determine the location and size of a crack in a solid. A characteristic function, which defines the size of a crack, can be reconstructed from the inverse Fourier transform of scattered amplitudes at far field. The inverse method is applied to ultrasonic data scattered by a crack in an aluminum specimen. Agreement between reconstructed characteristic functions and exact ones is not good enough, because experimental conditions do not coincide with theoretical ones. We can, however, evaluate the location and size of a crack from sharp minimum points reproduced at crack tips.