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TOKYO, July 18 (Reuters) - Shares in Tokyo Electric Power Co. <9501.T> fell 4 percent on Wednesday, their biggest drop in five months, on concern the shutdown of the world's biggest nuclear power plant after a strong earthquake will add billions of yen to its costs.

TEPCO has been ordered by regulators to keep the plant closed until its safety can be confirmed after a string of quake-related problems, including a radiation leak. (for story click [nT265821]).

The firm, Asia's largest power producer, has begun seeking power supplies from other utilities to cover its commitments [nT136369].

Closing the entire facility could cost TEPCO about 37-38 billion yen ($304-312 million) per month, analysts at Nomura Securities said.

On Tuesday, Mizuho Securities cut its estimates for TEPCO's operating profit by 10 percent to 520 billion yen for 2007/08, citing the impact of the plant shutdown.

That is below the company's profit expectation of 530 billion yen, and the market consensus of 549.5 billion yen based on a poll of 12 analysts surveyed by Reuters Estimates.

TEPCO shares lost 4 percent to 3,600 yen, shedding 195 billion yen from its market value and taking its losses since the quake to 5 percent.

The daily trading volume reached 13 million, more than triple the average over the past 30 days, Reuters data showed.

But some saw a buying opportunity in the stock. Mitsushige Akino, chief fund manager of Ichiyoshi Investment Management in Tokyo, said he expected TEPCO's share price to rebound as economic growth remained stable and strong demand was expected going into the peak summer season.

"There are some unclear factors such as the total costs on repairs or inspections and the timeframe of the reactor shutdown," Akino said.

"But the overall business environment for TEPCO is strong, so I think today's decline is temporary, and possibly a strategic one for some investors."

The 6.8 magnitude earthquake in northwestern Japan killed nine people, flattened hundreds of homes and sent more than 12,000 to evacuation centres. ($1=121.86 yen)

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