Travel 3 to 6 Months in Japan.

I'm planning to spend anywhere from 1 to 6 months in Japan. I intent to get digs for the first month and while I'm there wing if for accoms while I go from one end of the country to the other.
What I was wondering is what will I need to show customs on the way in, as I'll only have proof of address for the first month and a one-way plane ticket.
I gather I have to show proof that I've got enough money to fly home but will I need to convince them that I'm not going to disappear of end up working in a bar somewhere (unlikely as i'm 50).
I've had trouble convincing the Canadians that, as a single traveller and spending more than a couple of weeks in the country, i'm not going to disappear. I'm just trying to avoid this, this time.
Any pointers would be appreciated. Oh, I'm a Brit in the UK if that's useful to know.

What kind of visa do you intend entering on and what is the maximum period of stay it allows?

don't need one for the first 3 months. apply for an extension at a local immigration office after that.

Under visa waiver UK citizens get 90 days. This can be extended in-country for another 90 days (note this is 90 days not three months, and they do care about the difference). I don't know the exact procedure but I imagine it involves paperwork and tedium. You should figure what exactly that takes well in advance so you know where you have to be and how long before the end of your first 90 days you need to get it done.

This is the advice from the Japanese embassy in the UK on arriving on a one-way ticket:

Q: Do I need to show my return flight ticket to the immigration officer at border control when I wish to enter Japan as a tourist?
A: We do not have the authority to answer questions regarding the necessary documentation for immigration inspection because it remains at the discretion of the Japanese Immigration Bureau, Ministry of Justice. We strongly advise, however, that you carry documentation proving your intention to leave Japan when you finish your stay, such as a return flight ticket, as well as documents proving your planned activities and funds for your stay to ensure that the immigration inspection proceeds as smoothly as possible.

Click to expand...

So, either show a return/onward flight (doesn't need to be back to the UK), or be able to convince them that you can and will buy one before chucking out time. No guarantees. If I were you, I'd at least try to narrow my plans down a bit so you give the impression that you have a general idea of where you're going to be when and just want to leave the end date a bit open.

"So, either show a return/onward flight ..."
Thought about that. Would that be enough? I'm trying to keep my options as open as possible without looking too vague/like I'm up to something.

Reputedly some people deal with it by buying a ferry ticket to Korea dated appropriately. This may also help if the airline decide to be difficult about letting you on the plane (they are financially liable if you get turned around at immigration), which doesn't just happen for Japan.

Since you're spending so long in the area, it almost seems a shame not to take a side trip to Korea at the end - it's not far to go, and you can get direct flights back to the UK.

You also might consider talking to a RTW flight specialist about flexible tickets. I'm not sure how far you can stretch the flex (moving it months might be an issue). I say an RTW travel agent because they deal with complex trips where itinerary changes are often wanted or needed. I only bother with travel agents for multi-stop trips but they can be a life-safer. I think the ones I've used are "Round the World Flights" (imaginative!).

e.g. you could book an open jaw, into Japan out of Korea, and then get yourself from one to another by ferry or prebook the Japan-Korea leg as a flight on a flexible ticket. Watch the potential cost as even on a "flexible" ticket, if the flight you're moving to costs more than the one you were booked on you pay the difference.

Thanks, Neko. I'll thank you in advance because, TBH I don't fully understand everything you said.