* Real estate stocks extend rises after BOJ's decision, while banks continue falling

* Nisshin Steel soars as it considers alliance with Nippon Steel

By Ayai Tomisawa

TOKYO, Feb 1 (Reuters) - Japan's Nikkei share average soared to more than a three-week high on Monday as the Bank of Japan's surprise decision to adopt negative interest rates spurred further buying, though most economists doubted it would give much of a boost to the economy.

The Nikkei rose 1.2 percent to 17,727.62 points by midmorning trade, after hitting as high as 17,821.37 earlier, the highest since Jan. 8.

Traders said the BOJ's implementation of negative interest rates announced on Friday was a welcome psychological boost for global investor sentiment and business confidence.

It has also reversed some recent strength in the yen, though economists are unsure how long that weakness will last.

"Given the uncertainty over China and the uneasiness as the U.S. Federal Reserve begins a phase of interest rate hikes, the recent market sentiment fear is probably more of a crisis of confidence," said Kok Wei Yee, a portfolio Manager at Fidelity Worldwide Investment in Japan.

"The BOJ is well aware that significant market turbulence can negatively affect the real economy, and the easing measure is more of a pre-emptive and symbolic move."

Still, traders said it would be a big challenge for the Nikkei to recover its high of nearly 21,000 points seen last year.

"The BOJ's decision sent a message across the market that it will try to stop a slide in the market at any cost. But unless there is a sign of recovery in the real economy, the Nikkei's gains should be limited," said Yoshinori Shigemi, global market strategist at JPMorgan Asset Management.

Real estate stocks, beneficiaries of lower interest rates, extended their gains. Mitsubishi Estate Co rose 2.2 percent, Mitsui Fudosan Co surging 5.2 percent and Sumitomo Realty & Development climbing 3.7 percent.

Conversely, bank shares were again hammered as negative interest rates will eat into their interest revenues.

Mitsubishi UFJ Financial Group fell 6.8 percent, Sumitomo Mitsui Financial Group stumbled 7.2 percent, and Japan Post Bank dived 5.8 percent.

Exporters were in demand, with Toyota Motor Corp rising 1.5 percent, Honda Motor Co adding 1.2 percent and Panasonic Corp climbing 2.4 percent.

Nisshin Steel Co soared 18 percent after the company said that it is considering an alliance with Nippon Steel & Sumitomo Metal Corp including possibly becoming its unit.

The broader Topix added 1.8 percent to 1,457.28 and the JPX-Nikkei Index 400 rose 1.9 percent to 13,162.63.

(Editing by Kim Coghill)