Joining TPP would be good on so many levels, both economically and politically, that failing to do so would be utmost foolishness for Japan. It's obvious that Japan could use all the help it can get, economically as well as politically, to remain strong among the growing Asian economies. And the sooner it gets this help, the better.
Besides making markets more accessible to struggling companies, TPP would lower raw material and other input costs for those and other companies in the country increasing Japan 's competitiveness. Membership would also raise Japan's international standing, giving the country more clout in it's bid for a UN Security seat. Joining the partnership could also very well lead to other cooperative ties such as a Pacific Treaty Organization that might lessen Japan's dependence on the US military.
There are all these upsides to TPP that Japan can't afford to lose this chance to become a key member. And actually, it was a Japanese economist, Kojima Kiyoshi, who first proposed a Pacific FTA in 1966. His ideas were reiterated by PM Ohira Masayoshi in 1978.
I think Japan knows it needs this TPP. All that's necessary now is a modern day Ryoma Sakamoto who'll push it through. Of course, it shouldn't be the only economic bloc that Japan joins, but it should be one of them. It would lead to balanced growth.