Just in case it helps.
I have been working in Japan as an IT guy for around 10 years. Of course, in the IT world things changes a lot from one year to another, but this is my experience as developer:
- First job in Japan: around 4M (mostly PHP, some Java also)
- Changing companies almost every 2 years, in every change the salary increased around 1.5M (sometimes more, sometimes less, that's the average). With that "rule", if you have been working 5 years in IT related, you should be doing something near 7-8M.
- At this moment, with some good experience and working "more or less normally" in Japanese in a Japanese company, around 10M in senior development positions (aim to 12M if for lead development - team management level).
I have no idea if the situation is the same in foreign companies although I heard salary is better (I'm sure holidays and such are much better, getting only 10 days a year is ridiculous).
I also didn't plan the 2 years changing thing, it just happened that way for different reasons. But I keep an eye on how the market is and try to move that way for not becoming outdated (moved from PHP to Ruby and recently Golang). Studying Japanese is a must, but again, my experience is in Japanese companies.
A good place to check is
http://jobs.forkwell.com, is a job hunting site but has an uncommon salary range information for each position, it pretty much matches my experience, seems pretty accurate.