* Deal should be completed by April 2013
* Delay due to regulatory approval overseas
* Olympus shares down 1.8 pct, Sony shares flat (Adds Sony comment, shares)
TOKYO, Dec 21 (Reuters) - Japan's Olympus Corp and Sony Corp said on Friday that a planned merger of the two firms' medical business will take longer than expected due to delays in obtaining regulatory approval abroad.
Shares in Olympus fell 1.8 percent on the news after it traded in positive territory, against a 0.3 percent decline on Tokyo's benchmark Nikkei. Sony shares were off 0.1 percent.
Sony spokeswoman Saori Takahashi said the companies are working to establish the venture by April 2013, but did not specify the countries where the firms were facing delays in regulatory approval.
Sony said in September that it would pay 50 billion yen ($590 million) to become the biggest shareholder in Olympus and planned to establish a company by the end of the year with the cash-strapped camera and endoscope maker to develop medical equipment.
The deal, which was supposed to be completed by the end of December, gave a much needed boost to Olympus, which booked a net loss of 49 billion yen last fiscal year after a decade-long $1.7 billion accounting scandal was exposed in October 2011.
A Taiwanese banker was arrested in Los Angeles earlier on Friday, accused of helping to "liquidate" hundreds of millions of dollars for Olympus and having a direct role in the company's investments.
($1 = 84.39 Japanese yen) (Reporting by Tokyo Newsroom; Editing by John Mair and Matt Driskill)