This paper investigates whether Tobin tax would be effective to reduce exchange rate volatility. When the rational speculators observe different temporarily shocks and take Tobin tax into account, the exchange rate will have either stable or unstable path through speculators’ changing optimal holdings. If the effect of current account shock dominates the effect of interest differential shock, the imposition of tax will stabilize the currency. This result is consistent with Tobin’s view. On the contrary, if the effect of interest differential shock dominates the effect of current account shock, the imposition of tax will destabilize the currency. The best policy in this case is to let international capitals move freely.

無 / This study establishes a model of the stock market involving the rational speculators to investigate whether the imposition of tax on capital gains can reduce the market volatility. The finding is that the effect of tax on the stock price volatility varies according to the types of shocks hitting the market. In the cases of the issuing shock and the dividend shock, raising the tax rate could be a way to stabilize the stock market. On the contrary, when the margin-rate shock occurs, it tends to magnify the effect of the shock and therefore increases the market volatility. Thus, it could be concluded that an increase in the tax rate may increase or decrease the stock price volatility depending on the type of unexpected shocks.