Judge Jumpol na Songkhla said: "The court voted eight to six to reject the petition."
He was speaking after the court met to debate allegations by 28 senators that Thaksin Shinawatra controlled Shin Corp despite handing over his shares before taking office.
The allegations, which included charges that Thaksin tailored policy to help the business empire he founded, were not detailed enough to warrant an investigation and the senators would have to elaborate in a new petition if they wanted it heard, Jumpol said.
The ruling was booed by 100 anti-government demonstrators.
But it gave a quick boost to a nervous stock market, where the main index was up 1.01% at 733.04 points at the lunch break.
Shin Corp stock, which fell before the court verdict, was up 0.52% at 48 baht.
Kaewsan Atipoti, one of the 28 senators, said his group would fight on, but declined to say how. "This is not beyond our expectation. We already have Plan B," he said.
Thaksin, a billionaire and winner of two historic landslide election victories, is accused of weakening the checks and balances enshrined in the constitution. He had shrugged off the attempt to oust him.
"No worries, no problem," he told reporters this week when asked about it.
Thaksin's support in the
countryside seems solid
Thaksin's support in the countryside, where 70% of Thais live, appears to remain solid.
But the financial markets had been nervous after two big protests against him in Bangkok, where middle-class people were outraged by the tax-free $1.9 billion sale of 49.6% of Shin Corp to Singapore state investment firm Temasek last month.
Foreign investors, who bought Thai shares as the market rose solidly throughout January, turned into sellers as the main index dipped steadily in recent days.
Thaksin had previously dismissed allegations similar to those in the petition and Shin Corp has brought criminal and civil defamation actions - claiming a total of $10 million in compensation - against others who made such charges in the past.
Thaksin said at the time of the Shin Corp sale - which also stirred up worries about a sale to foreigners of assets, such as satellites, potentially vital to national security - that killing off such allegations was one reason for the sale.
If the court had accepted the petition and its investigation had substantiated the charges, Thaksin would have been dismissed from office for breaking the constitution.
Thaksin, a former policeman who accumulated a fortune in telecommunications before entering politics, has been accused of shady practices before.
He won a razor-thin court verdict against charges of illegal asset concealments in August 2001, which would have excluded him from office.
Then, the Constitutional Court ruled by eight votes to seven in favour of Thaksin after he was indicted by the anti-graft body of trying to conceal assets by transferring them to security guards, drivers and maids in the 1990s before becoming a cabinet minister.
Thaksin said he had made an "honest mistake" and had no intention of concealing his assets.