TOKYO, Dec 17 - Japan's top financial diplomat, Zembei Mizoguchi, reiterated on Wednesday that Japanese authorities are ready to take action against big swings in the foreign exchange market. "There are times when the market reflects fundamentals, but when there is excess volatility and overshooting, we take appropriate action," Mizoguchi, vice finance minister for international affairs, told reporters as he arrived for work. The dollar was trading at 107.48 yen , steady from its New York close.//