Kasumi is a big chain of supermarkets in Ibaraki.
The best time to shop at Kasumi is at the weekends. Not only do they lower the prices of lots of items called koukoku no shina, they also give away lots of free samples, so your kids can stuff themselves while you're not looking.
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In Tsukuba they have branches in
There are also branches in Ushiku and Tsuchiura.