As long as you find yourself in deep darkness, away from both strong moonlight and from light pollution from residences and streetlights in particular, you should be able to see it. A good pair of binoculars might work best in order to spot it.
It should be visible at approximately 11 degrees above the horizon in Tokyo in a northeastern direction and at approximately the same height and direction throughout the rest of Japan.
Currentlly, Japanese astronaut Furukawa Satoshi has been aboard the ISS since June 10. One of the experiments that he is attempting involves growing cucumbers under weightless conditions. If you are lucky, Furukawa-san may perhaps take some time off from his gardening chores to wave at you as the ISS sails past.
Generally the ISS will be visible for five or six minutes at most, and in some cases, for as little as two minutes.
After this limited window of opportunity during the Perseid meteor shower, the ISS will not be visible again until next month.
Here are the times for Tokyo, which are also good for the rest of Japan.