The regulations can be changed without putting domestic farmers out of business. For example, they could add tariffs to imported potatoes to make them cost as much as (or even more than) domestic potatoes, and use the revenue to help Hokkaido farmers rebuild their farms. No need for a hard limit on the imported amount.
If there is political will, things like that can be done remarkably quickly.
South Korea had a massive shortage of chicken eggs last winter due to an avian influenza epidemic. It took only a few weeks for the government to start importing fresh eggs from abroad, something that had never been done before. It wasn't enough to get prices down to normal levels, but at least the government acted quick -- especially considering that the nation was in the middle of a Presidential scandal.
Unlike South Korea, Japan is essentially a single-party dictatorship. They can pass whatever laws they deem necessary if they really want to. I'm assuming that they don't want to, probably because there's someone who profits from high potato prices.
It wasn't that long ago that the DPJ held power. Anyway, that's a fair point, but I think a shortage of eggs where people eat a lot of eggs and a shortage of potato chips are kind of different situations that don't necessarily need the same kind of response.
