TOKYO (Reuters) - McDonald's Japan <2702.T> has apologized to customers and vowed to ensure product safety after objects including a tooth and plastic were found in its food, the latest trouble for a fast-food chain hit by sliding sales and a shortage of french fries.

The chain, 49.9 percent-owned by McDonald's Corp , has been grappling with falling sales that began long before a food safety scandal last summer hit confidence in its products. Convenience stores in particular have drawn away customers with broader ranges of ready-made meals and low-priced coffee.

McDonald's Japan brought in Chief Executive Sarah Casanova last March to reverse the trend, but the company is still on track to record its sixth straight year of sales declines and its first annual loss in 11 years.

Wednesday's apology came after a diner found a roughly 4 cm (1 1/2 inch) strip of vinyl in a Chicken McNugget at the weekend. That prompted the chain to halt sales of nuggets made on the same day as the contaminated item at a plant in Thailand. The company is still investigating the cause.

Among other incidents, a human tooth was found in a customer's french fry in August, while a child in December cut his mouth on a piece of plastic that was in a chocolate sundae.

At a packed news conference, executives sought to reassure the public of the safety of McDonald's food.

"I am confident that my family can eat McDonald's products," said Takehiko Aoki, senior vice president at McDonald's Holding Co (Japan) Ltd. "I think our response has been appropriate," he said when questioned on whether the company had been slow to announce its findings, explaining that the handling of each case is different.

McDonald's Japan only started sourcing nuggets from three Thai plants less than six months ago. The change was aimed at boosting confidence in product quality after Chinese supplier Shanghai Husi Food Co Ltd was accused of selling expired meat.

The Thai plant that produced the nugget found to contain vinyl is located in Saraburi, owned by Cargill, McDonald's Japan said. Cargill Thailand was not immediately available for comment.

On Monday, just days after the latest discovery, McDonald's Japan resumed sales of all sizes of french fries for the first time in three weeks. A labor dispute at U.S. West Coast ports had delayed imports, forcing the company to ration portions. McDonald's Japan shares closed down 0.9 percent on Wednesday versus a 0.3 percent rise in the Jasdaq index <.JSD>.

By Chris Gallagher

(Additional reporting by Chris Meyers in TOKYO and Khettiya Jittapong in BANGKOK; Editing by Christopher Cushing and Elaine Hardcastle)