Is it better to get a pre-paid SIM card, mobile wifi hotspot, or rely on provided hotel/restaurant/public free wifi?
I highly recommend you to rent a mobile wifi or a prepaid SIM. You won't find much free wifi area in Japan. You should be able to get it at Narita International Airport
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Thanks guys! I ended up buying a prepaid SIM (data-only) online and am having it delivered to the address of the friend's apartment where I'm staying. It ended up being $30 for two weeks of data. It seemed super easy, I can pass anyone the link if they're interested!
Surprisingly, public wifi is scarce in Japan. I find it harder to connect to a free wifi public hotspot there than in emerging countries.
It makes more sense to have a wifi on the go when you are traveling in Japan since train lines are pretty confusing. And for some reason, you can't download offline Japan Google maps.
I suggest you to rent a pocket wifi in Japan. It is not that expensive actually. You can rent it for USD 50 - USD 60 for one week sharable up to 10 devices at a time.
Free Wi-Fi is not widely available. You can get it in starbucks or McDonalds but there is no public free wifi. It can also be difficult to get a SIM card in Japan because they often require a residence card, but there should be some options at the airport. You may be able to get a data only card if you have a smartphone and that's probably your best bet.
Its quite difficult to get a Japanese sim card in Japan. When I lived there I had to prove I had some kind of visa or residency card to purchase a Japanese sim card. Even then, I couldn't just buy a sim card, I had to buy the cheapest phone that it would work with. Unless you can speak Japanese it will probably be difficult to get a Japanese sim card.
However, for wifi, you can use the starbucks all over Japan. Sign up to their wifi first from a computer, then you'll be able to log in when in/outside the starbucks.
General advice is to not rely on free-wifi because it isn't always that good (despite more than 100,000 spots across the country).
Getting a data sim is easy - a voice sim isn't really necessary and as mentioned in a different answer, hard to get (misuse of telecoms law comes into play for voice).
Portable units work and from 300yen/day for 10 devices and high data speeds, can be a very good option.
in japan. many shops has free wifi.
but holding pre-paid SIM card is better for your trip for moving smoothly.
When I traveled to Japan, I rented a "Mi-Fi" mobile hot-spot which I highly recomment. The wifi in the hotels we stayed at was pretty good, but you're right the public Wifi is scarce. The mi-fi mobile hot spot (ours was made by Docomo.) is awesome. Your entire party can use it if you're in the same close vicinity, even while walking around the city, or in your hotel if they charge a fee for wifi, so pricewise since each person can use their own mobile phone on wifi mode it works out to be even more reasonable. I used Telecom Square. Since I flew ANA Airlines , they had a deal where you got a discount on the mobile hot spot rental too. Google mobile wifi hotspot rentals Japan and you'll find a couple of others. Telecom Square sent me the hotspot equipment at home too, so I had it before I got on the plane. Also if you have Japanese markets nearby, there are, at least in the USA, usually telecom companies inside them that you can rent mobile hot-spots through. I have to admit the 4G wifi was pretty fast on the Docomo. Hope that helps.