The partial analysis method is developed to solve the scattering problem of Lambwaves by a finite crack in a plate. First, the scattering coefficients of Lamb waves due to a semi-infinite crack are obtained by using the mode-exciting method. Then we consider the equivalence between scattering coefficients of Lamb waves by a finite crack and those by a semi-infinite crack to obtain a system of equations depending on the crack length explicitly. The partial analysis method has an advantage in computational time when several plates with single cracks of various lengths are to be analyzed. Comparison between the results of the partial analysis method and the direct mode-exciting method shows a good agreement. The application of the partial analysis method to an inverse analysis is also demonstrated.