When grapes get to a certain size they need sunlight and small amounts of rain to ripen. If you get too much rain, their skin splits (just like cherries). If they don't get enough sunlight, they never sweeten properly. Too wet + no sunlight = poor quality grapes.

Grapes like cold winters, plenty of moisture in the spring followed by intense heat with minimal to moderate amounts of rain in the summer (think climate of Southern France/California...you get the idea). It sounds like Japan's climate is not something that works well for grapes.

What you are seeing is normal for grapes in your climate. You can consider bringing in the bunches before they split. They might be saved by bringing them in from the rain. There is no way to stop their roots from gorging on ground water. That is what is causing the skins to split = too much ground water fills the skins to bursting (literally).

Thanks very much!