Read this: Although the ECB is governed by European law directly and thus not by corporate law applying to private law companies, its set-up resembles that of a corporation in the sense that the ECB has shareholders and stock capital. Its capital is five billion euros which is held by the national central banks of the member states as shareholders. The initial capital allocation key was determined in 1998 on the basis of the states' population and GDP, but the key is adjustable. Shares in the ECB are not transferable and cannot be used as collateral.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What governs the ECB?
European law