Before you start

Residence card with address printed
A Japanese bank account or credit card, for most postpaid plans
An unlocked or eSIM-capable handset that supports Japanese emergency broadcasts
A Japanese address, for fibre

Step-by-step

1
Use a travel eSIM for the first days
A data-only eSIM bought before arrival covers you until the residence card and bank account are sorted, which can take a fortnight. It removes the pressure to sign something unsuitable in week one.
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2
Confirm your handset receives the Earthquake Early Warning
Domestically sold Japanese handsets receive the national Earthquake Early Warning and J-Alert cell broadcasts by default. Some imported models do not, or have the setting buried and switched off. Check it on day one and leave it on — this is the alert that gives you the seconds before shaking arrives, and it is the reason people here do not silence it however irritating it is.
Mobile appWho: YouDay 1
3
Go straight to an MVNO or a carrier sub-brand
IIJmio, mineo and the carriers' own low-cost brands run on the same networks as the majors at a fraction of the price, with no long commitment.
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4
Expect a Japanese payment method to be required
Most postpaid plans require a Japanese bank account for direct debit or a Japanese-issued credit card, and a foreign card is frequently rejected. This is why the bank account has to come before the phone contract.
OnlineWho: You
5
Order home fibre early and ask what reaches the room
Japanese fibre is fast and inexpensive, but installation in an apartment building depends on what is already wired in. Some buildings run fibre only to the basement and older copper to each flat. Ask the landlord or agent before signing if your work depends on it.
OnlineWho: You
6
Keep your number when you switch
Japan operates mobile number portability and has simplified it considerably. Never cancel the old contract before completing the switch.
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Documents you’ll need

Residence card with address
Japanese bank account or credit card
Passport, occasionally requested
Proof of address, for fibre installation

Things most newcomers don’t know

An imported handset that cannot receive the Earthquake Early Warning is a genuine problem here.

Japan's cell-broadcast warning system gives anywhere from a few seconds to a minute of notice before strong shaking arrives, and in a city on this coast that is the difference between being under a table and not. Domestically sold phones do it out of the box; some imported ones need the setting enabling and a few cannot do it at all. Check it before you rely on the phone, and check it again after any handset change.

Source: Ministry of Internal Affairs and Communications

The bank account gates the phone, not the other way round.

Most Japanese postpaid plans require a Japanese bank account or Japanese-issued credit card, and foreign cards are commonly rejected outright. Since the bank account requires ward registration, and ward registration requires an address, the chain runs address → ward office → bank → phone.

Source: community-reported

MVNOs run on the same towers for a fraction of the price.

Government pressure on the three major carriers produced both low-cost sub-brands and a healthy MVNO market, all using the majors' own networks. Coverage and speed are essentially identical outside peak congestion — though in the mountains west of the city and along the Sanriku coast, the major carriers' own coverage maps are worth checking rather than assumed.

Source: Ministry of Internal Affairs and Communications

An apartment's 'fibre' can be shared copper from the basement.

A building advertised as fibre-connected may run fibre only to the building and older wiring to each flat, capping your real speed well below what the plan promises. Ask specifically whether the connection is fibre to the room.

Source: community-reported

Common mistakes to avoid

Carrying a handset that never receives the Earthquake Early Warning, or switching the alert off.
Trying to get a phone contract before ward registration and a bank account.
Signing a flagship carrier plan when an MVNO on the same network costs a fraction.
Assuming a building's advertised fibre reaches your actual flat.
Cancelling an old contract before completing the number transfer.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.