Under a zero interest rate, the cost associated with holding either banknotes or the central bank’s current deposits is negligible; no matter how much money it supplies, this will be held in such a form as the central bank’s current deposits. In terms of quantity, this is completely ineffective.
In fact, Chairman Bernanke has stated that the degree of monetary accommodation cannot be measured properly based on the quantity. Having said that, the fact is that the quantity itself is the highest in Japan.