They also appear in other Japanese airports. What do they mean?
I dug around for a while but couldn't find any official documentation on this (I suspect someone whose Japanese is better than mine will have to do that.)
But! I did find the markings on all of the Haneda Airport runways and I noticed they were roughly halfway along the length of the entire physical runway. A little bit of measuring confirmed my suspicion, it appears the marks show you exactly where the halfway point is on that particular runway.
I also looked at other Japanese airports and took measurements of the same markings at Narita, Osaka and Kobe. All airports turned up the same results. (You can use the measure tool on Google Maps to confirm if you'd like to)
I also find out that not all airports in Japan have these markings. Neither Nagasaki nor Osaka seem to have markings like the ones mentioned above...
Not sure why they mark the halfway point...someone with better Japanese than me will have to look into the history of that decision.
Edit: mins found some documentation and posted it in the comments (thanks mins!):
This is an English translation of a report done by the Japan Transport Safety Board regarding a runway overrun. They mention the meaning of the markings (in passing) on page 2.
So, there we go, more than just circumstantial evidence we have the aviation authority of Japan saying it's true. It's the halfway mark for the runway.