I'll steal a phrase from Adamu to describe this.
NTT Docomo will begin taking preorders for the iPad Air and iPad Mini Retina on June 2nd, and the devices will go on sale June 10th.
With the new pricing structure of Docomo data-only plans, where 5GB now costs ¥7,000 when 7GB used to cost ¥6,000, there is really no reason at all to buy a tablet from Docomo unless you are bundling with a number of devices.
Even so, the minimum cost of adding each additional device is ¥2,400 (slightly more if you don't hack the device and want to tether.)
You are much better off picking up a fully unlocked iPad and trying your luck overcoming Apple's locked down APN settings to get an LTE connection with an MVNO.
Docomo's iPad page.