This paper describes a method for estimating, using many sensors, the position of a crack in a concrete block, most of which is buried in the ground. An array of sensors is attached on the surface of the concrete block, and a vibration pulse is forced using a small hammer. Since the forced vibration reflects at the position of a crack, it may seem that the position can be easily estimated by picking up the reflection wave with an array of sensors and by analyzing the received signals using a beamforming technique. It is difficult, however, to estimate the position since the magnitude of the surface wave is much greater than that of the reflection from the crack and there are many reflection waves from the boundaries of the concrete block. Therefore, this paper proposes a method to eliminate not only the effect of the surface wave but also that of the reflection waves from the sidewalls and/or bottom, and extracts the necessary reflections (primary and secondary waves) from a crack. The position of the crack is estimated using the extracted waves by a beamforming technique.