* Outperforming Topix 30 triggers pension buying speculation
* Short-sell ratio relatively high
* Takata dives on report bidders consider possibility of some form of bankruptcy proceedings
By Ayai Tomisawa
TOKYO, Sept 20 (Reuters) - Japanese stocks rose in choppy trade on Tuesday morning as investors counted down to this week's Bank of Japan policy decision amid speculation the central bank will make crucial changes to its massive easing programme.
There is talk the BOJ could make negative interest rates the primary focus of its monetary policy on Wednesday as policy makers seek new ways to stoke stubbornly low inflation and growth.
The uncertainty around the BOJ's decision has kept markets on edge on top of the looming outcome of the Federal Reserve's review which also falls on Wednesday.
"The market is more nervous about the BOJ than the Fed as it may give a surprise," said Yutaka Miura, a senior technical analyst at Mizuho Securities.
He noted the market consensus for the Fed is for rates to stay on hold.
"There is a lot of speculation for the BOJ but there is no consensus," on what the Japanese central bank will do, Miura said.
The Nikkei gained 0.3 percent to 16,570.16 in midmorning trade after opening at the day's intraday low at 16,403.22.
But the broader Topix outperformed, rising 0.9 percent to 1,322.66.
Bellwether stocks outperformed the market, with traders speculating that pension funds may be buying components of the Topix Core 30 index, which soared 1.2 percent.
"Exporters are bought although the yen is not particularly weak, so it's natural to assume that pension funds started buying after the Nikkei was seen as cheap below the 16,500-mark," said Norihiro Fujito, a senior investment strategist at Mitsubishi UFJ Morgan Stanley Securities.
Toyota Motor Corp gained 2.2 percent, Nissan Motor Co rose 1.6 percent and NTT Corp soared 3.0 percent.
Traders also said that investors appear to be covering their short positions before the BOJ and Fed reviews.
As of last Friday, the short-sell ratio stood at 39.7 after hitting 44.9 on Thursday, the second highest after a record high of 47.1 hit on June 10.
Bucking the trend, Takata Corp dived 14 percent after a Bloomberg report that some of the company's bidders are considering the possibility of some form of bankruptcy proceedings for the troubled Japanese air bag maker.
The JPX-Nikkei Index 400 gained 0.8 percent to 11,868.39.
Editing by Shri Navaratnam