Slingbox alone will not work.
Slingbox antenna/coax input only supports ATSC/NTSC/QAM transmission. However Japan uses ISDB-T transmission, so you need to get an ISDB-T tuner. Additionally, since Slingbox does not support HDMI-in, you will need to get an ISDB-T tuner which provides component output for HD picture, or S-video output for SD picture; or optionally, an HDMI-component cable adapter.
[hide]When you buy a TV in Japan, it automatically includes (typically) dual ISDB-T tuners for viewing 地デジ/BS/CS, however TVs critically do not have
video output (only audio output for home theater speakers), which is why you must buy a separate ISDB-T tuner in order to send the video signal to a Slingbox.[/hide]
Here are some popular standalone ISDB-T tuners:
http://buffalo.jp/products/catalog/multimedia/chideji/tv-chideji.html
http://www.iodata.jp/product/av/tuner/
And a more comprehensive list of standalone ISDB-T tuners:
http://www.coneco.net/SpecList/01801030/
If you wish to watch BS/CS subscription channels, many Japan ISPs now offer IP-based BS/CS access. They will send you a new ADSL/cable/fiber optic modem which includes both ethernet port (for traditional internet access) as well as HDMI, component, or composite plugs (to hook up to your TV or Slingbox) for watching 地デジ/BS/CS.
You will need to set up the IR blaster on the Slingbox in order to change channels, because the Slingbox itself cannot change channels (you must change channels on the ISDB-T tuner).
Additionally, you will need to enable port 5001 in order to connect to your Slingbox from abroad. You will have to make sure your ISP allows external access on port 5001.
Lastly, because 地デジ/BS/CS transmissions are DRM-protected, you must confirm any content-protection limitations. Some tuners may automatically drop down to 720 HD or 480 SD if they cannot confirm HDCP, so you will have to ensure all of your devices will support whatever signal the tuner will output. There are a few DRM-free ISDB-T tuners (in particular, Friio and PT2), but they are PC-based tuners (not standalone boxes) and will require additionanl software to configure.
