The Japanese Celiac / Coeliac Gluten Free Restaurant Cards mentioned above are surely very useful but the text is so pixelated that I could barely read the Japanese, so I remade it as best I could. I input the English and Japanese text and made a PDF that people can download and print out for your travels to Japan.

Printer Friendly Celiac Disease English-Japanese Card PDF
Celiac Disease English-Japanese Card PDF
Celiac Disease English-Japanese Card PDF (text outlined version)
If your computer has trouble rendering the Japanese text in the PDF, the outlined version ought to work for you.
I don’t have Celiac Disease and don’t know anything about it. I hope this is helpful for people. Please let me know if there is anything that needs changing, fixing or editing.