In Japan, it is known that a doctor's bill is so expensive even for people having public insurance, therefore for travellers to Japan, it is recommended you take out a traveler’s insurance policy.
If your illness is not so serious, you can go directly to a drug store to get over-the-counter medicine, and then stayi in bed to care for yourself. Pharmacies or Drug stores are called “Doraggu Sutoa” = Drug store in Japanese. To find Japanese phrases to use when buying medicine at a drug store, see eigo-samples.com
If you feel your illness is serious and need to go to a hospital immediately, you can search for hospitals in Tokyo on this web site.
For medical emergencies, it is recommended to ask someone who speaks Japanese and call an ambulance (119). Hopefully, calling up an ambulance will be final choice as you might be taken to large hospital far from the place you stay.
[ Related web pages ]
Public Health Insurance Systems in Japan