For those with a hankering for real deal Japanese while in the Big Mango, I highly recommend Ten Sui on Sukhumvit 16. The restaurant is in its own house with a large courtyard/garden. Step into the property and you feel like you are in Japan. They have regular seating and tatami rooms. Note: this is a high level joint and priced accordingly for some things, but I would match the quality against just about any Japanese I have eaten outside Japan itself.
They cover all the basic Japanese "sub-cuisines" and specialize in Kaizeki and v. fresh sashimi/sushi. Those two will set your wallet back - you are starting at 3000 baht for Kaizeki or 2300 baht for Sashimi for 2 ppl. They are constantly changing the specials with the seasons and the ingredients are of highest quality.
HOWEVER, more staple dishes like assorted tempura (B350), Miso Cod/grilled salmon (B380/280), udon/soba (B250) and others are much more reasonable (relative to Western or Hong Kong prices and even the rising prices in Bangkok HiSo restaurants).
The lunch sets are a great option - starting from 330 baht you have a wide choice of lots of very good and very filling lunch sets.
As a high end place slacks, shoes, collared shirt are necessary.
As a comparison, I would rate this place far higher than the Japanese in JW Marriott (Tsu & Nami) at similar or lower pricing levels.
See the very extensive menu at their website: http://www.tensui.co/
Some good pics at their facebook: https://th-th.facebook.com/Tensui16
