Gluten-free in Japan

After all these years studying Japanese and wanting to get to Japan, now that it seems I may achieve my goal sometime in the near future, my main worry is food! I have been looking into it, and really have got nowhere. I know at least that Tamari soy sauce you can buy here in Spain (although quite difficult to find) is ok, but it seems to be Japanese don't really know what coeliac disease is... All I found on a forum is someone who read there were under 30 diagnosed Japanese coeliacs in 2006 (or was it 2005?). And the coeliac disease entry in the Japanese Wikipedia doesn't really have any links to anything useful in Japanese! I have found various posts in forums enquiring about where to eat gluten-free in Japan (especially in Tokyo), no-one seems to really have come up with a clear answer... except one Japanese person who said you can shop gluten-free in Natural House (and I'm afraid I'd already come to that conclusion thanks to an advert on a webpage... I'd be lost without Google ads!). The only problem is knowing what you can buy, as if it wasn't hard enough in your own language... luckily here's a video on youtube about forbidden ingredients....