In the first 18 weeks of both games being on sale, Mario Kart 8 sold 678k and Splatoon sold 681k. (not including digital) 

In Mario Karts 18th week it sold 6k, while Splatoon sold 20k. 

I'm kinda surprised but not at the same time. Before launch I would have said no way would it be a faster seller than Mario Kart 8, but based on this, it'll be the best selling Wii U game of all time in Japan. (not counting unreleased games yet, although Super Mario Maker might be able to)

http://wiiudaily.com/2015/09/splatoon-best-selling-wii-u-game-in-japan/

http://www.neogaf.com/forum/showpost.php?p=180341852&postcount=7

Man, Splatoon sure has become a success, most videogames wish they had half of  MK's legs, specially a console game in Japan.

Also, will the next Mario Kart have a Splatoon course?

Nice, good to see Splatoon doing really well in Japan. Next, it's a road to 1M

Now we just await the next milestone: 1m!

I still remember the days when I thought 200k in Japan was optimistic.

Nice to see such a success for a new ip in Japan. It proved to be exactly the game the wiiu needed too and has been driving it's sales ever since it released.

Cute squid lolis are cute and Japan loves cute.

Ryng must be proud, getting his graph used in an article.

Can you link my source, since this is my graph?
Thanks.
http://www.neogaf.com/forum/showpost.php?p=180341852&postcount=7

...Great thread paatar.

Really impressive

Is the game moving as much consoles as mario kart 8 did as well?

However, part of me thinks that this success is due to larger install base

The game is selling much more consoles in Japan than MK8 did. It has shifted the baseline entirely:

http://gamrconnect.vgchartz.com/thread.php?id=203794&page=1#

Larger install base is a factor, but it's also expanding the install base, which is equally important. (I mean, during Week 18, MK8 sold about 6k, while Splatoon sold 20k. The Wii U also sold about 18k that week in 2015, while in 2014 it sold a little under 10k. Quite a difference in both figures.)