A method and system for discovering dependencies in project plans of a distributed system. Each of the filter media is formed with a filtering area of 20 to 35 m2. The time during which the current flows in a first direction and the time during which the current flows in a second direction opposite to the first direction are converted into electrical signals and amplified. The magnetic shielding member covers the range in which the magnetic head moves, and is disposed in the cut-away portion so as to form substantially the same surface as the surfaces of the upper wall, side wall and bottom wall of the housing.