Intervention in currency markets is a bad thing: China's fixed exchange rate, for example, has created a dangerous global imbalance. But Japan now has the chance to get rid of the unnecessary foreign currency reserves it built up in previous interventions and boost the weak yen at the same time.

The yen now trades around Y120 to the dollar and has fallen even further against the euro. Its trade-weighted index is at an eight-year low. The falling yen upsets Japan's competitors because it makes their own industries less competitive. It also increases the risk of a sharp and disorderly rise in the yen at some point in the future.