This study exams a forecasting power of option volumes implicit in KOSPI 200 options by analyzing minute by minute historical index option intraday trading data from January of 2007 to January of 2011. We begin by calculating volume ratio of call and put for ATM, OTM and ITM options, and then compare the daily rate of return of the signal following trading strategy that we buy (sell) a stock index when the volume ratio increases (decreases) with that of an intraday buy-and-hold strategy that we buy a stock index on 9:05AM and sell it on 2:50PM. We found that the rate of return of the signal following trading strategy was significantly higher than that of the intraday buy-and-hold strategy, which implies that the option volumes have a strong forecasting power on the direction of stock market. Secondly, the stock index has a positive relationship with the ATM and ITM call volume but a negative relationship with OTM call volume which contradicts to the view that informed traders tend to prefer OTM options with higher leverage to the ATM. Another finding is that the information contents of option volumes disappear after two minutes.