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In Korea, Google Maps is limited in its functionality (due to protective legislation, I believe), and therefore, isn't very useful for non-drivers. For instance, it will not plot a walking route. You pretty much have to use the map apps from Naver or Daum/Kakao, both of which require knowledge of Korean, but are very detailed, even to the point of calculating taxi and bus fares for the possible routes.

Is Google Maps good enough for tourist* use in Japan? Or is there another commonly used map app?

*Tourists who cannot understand/use Japanese, who will be walking and using public transportation, who will not be driving, who might use taxis.

I have spent about fifteen months in Japan over the last few years (mostly Kansai) and I have never found a need to use anything except Google Maps for finding out how to get anywhere.

You can plot walking directions easily enough, at least in cities, and they have always been accurate when I have used them. Google Maps coverage of everything from buses, subways, and trains is also good and on point.

I have lived in Tokyo for 18 months now and Google Maps has served me well 90% of the time. It has complete bus, train, and subway service in large cities and it seems to know the speediest exit to take from most subway systems. I have never seen it make an error with regard to walking directions.

There are really only two exceptions to this. First, Google Maps does not know whether you should take a bus, shinkansen, or airplane between different parts of the country; you need to research this yourself. Second, for reasons that are probably algorithmic and which I can't describe in systematic detail, Google Maps is sometimes unable to find the absolute fastest or most convenient subway route inside Tokyo. For this, the best app to use is Jorudan's 乗換案内 but it is only available in Japanese.

If you live in a suburb of Tokyo and own a commuter pass, a three-month subscription to Jorudan's 乗換案内 is only 900 yen and can easily save you that much money by finding cheap, convenient routes that use your pass. There are several other apps that offer the same service (all Japanese only). One app in particular, NAVITIME, seems to have an English version aimed at tourists, and it might be handy for getting you from the airport to your hotel.

I have used MAPS.ME in Japan with good results. You can download the maps for offline use so you don't need data connection while traveling.

The vast majority of people in Japan use Google Map to navigate through the cities. Apple Map is infamous for being incorrect in many places. Other map apps exist (e.g. Yahoo! map), but the share of them would be likely less than 5%. As with the case in South Korea, most of such services developed in Japan are only focused on Japanese market and its people, and they rarely consider localizing it. I recommend Google Map.

That being said, sometimes Google Map may not take you to your destination in public transport in the whatever best way posssible, like fastest, least number of times of transfers, or cheapest. Local people usually use 乗換案内(norikae annai), though it is only available in Japanese language. There are similar transport search apps available, and I'm not surprised to see a few of them are localized in Korean and Chinese as well as English.

If you travel in Tokyo, I recommend CityMapper. Even some locals here prefer it to any Japanese apps since it is better in UI. This is only available in the central of Tokyo, and is not good outside of the central like a suburban of Tokyo including a route from Narita Airport, though.

Osmand Android app worked for us very well. It is possible to pre-download maps for the whole Japan in advance and then it works fully offline. It makes sense to put all planned hotels and points of interest at home and in advance, before heading to Japan. At least hotels usually have the location plan that is possible to correlate with the Osmand maps, Booking.com includes the map reference for every hotel.

You can also buy the data-only SIM card and then Google Maps become available. This app is also good. But you need recent unlocked phone (better dual SIM) and some tweaking of the phone settings (English instructions were included with the card).

GPS location worked surprisingly well in all Japanese trains and buses we used, providing perfect information where to get out.

We also tried multiple other apps but they performed significantly worse than the former two.