Two possible causes of international stock price co-movement are examined: the existence of global common shocks and portfolio adjustments by international investors. Empirical analyses indicate that the former explains a significant part of the co-movement and the latter is unlikely to play an important role. We extend the analysis to intra-day high-frequency data. For example, when the Tokyo Stock Exchange begins its daily trading at 9:00 A.M. Japan Standard Time (JST), stock prices in Tokyo exhibit responses to preceding changes in New York. An analysis with minute-byminute data indicates that Tokyo's response to New York dissipates within about six minutes after opening. On the other hand, when the New York Stock Exchange (NYSE) opens at 9:30 A.M. Eastern Standard Time (EST), its response to Tokyo dissipates within 14 minutes. Thus, the movement of stock prices is transmitted rapidly across countries. Finally real-time simultaneous interactions between Shanghai (Shenzhen) and Tokyo are analyzed for a 30-minute period in the morning and a 60-minute period in the afternoon. Investors in Tokyo are watching stock prices in Shanghai, but not vice versa. Tight regulations on Chinese investors to prevent them from holding foreign stocks may be the reason why they do not pay any attention to stock price movements in Tokyo.