* Windsor Quality acquisition would be biggest for Ajinomoto
* Aims for $1 bln frozen food sales in N.Amer by 2020
* Ajinomoto shares up 1.7 percent after news (Adds details on deal, context)
TOKYO, Sept 10 (Reuters) - Japan's Ajinomoto Co Inc said on Wednesday it would buy unlisted U.S. frozen foods maker Windsor Quality Holdings for around $800 million as it seeks to expand in North America to offset slowing growth at home.
The deal is Ajinomoto's most expensive acquisition to date, Thomson Reuters data shows. Like other Japanese firms, Ajinomoto is looking abroad as the domestic market shrinks due to an ageing population and a slowing economy.
Texas-based Windsor makes ethnic food brands such as Ling Ling and Jose Ole, and sells its products to 80,000 retailers and 120,000 restaurants in the United States.
The purchase is expected to be completed early November and Ajinomoto will finance the deal with cash and loans, a spokeswoman said.
Ajinomoto said in a statement that Windsor's marketing and distribution capabilities would help accelerate its growth in North America's $40 billion frozen food market.
The Japanese company, a household name in several Asian countries, said it aims to boost sales at its frozen foods business in North America to about $1 billion by 2020.
Ajinomoto has said it wants to grow its overseas business through acquisitions as it aims to become one of the world's biggest global food companies, with a market value on par with firms such as the United States' Campbell Soup and Mexico's Grupo Bimbo.
Earlier this year, media reports said Ajinomoto was interested in buying French ingredients firm Diana. German ingredients firm Symrise has since said it would buy Diana in a $1.8 billion deal.
Shares in Ajinomoto were up 1.7 percent at 1,743 yen in mid-afternoon trade in Tokyo, while the benchmark Nikkei average was up 0.3 percent. (Reporting by Chang-Ran Kim; Editing by Chris Gallagher and Miral Fahmy)