The new factory, to be located in the country's eastern province of Rayong, is scheduled to begin production by late 2009 with an initial capacity of 20,000 units a month.
Monthly production will eventually rise to 140,000 units, which would raise Ricoh's overall capacity by about 70 percent.
Ricoh, which competes with Canon Inc (7751.T: Quote, Profile, Research) and Xerox Corp (XRX.N: Quote, Profile, Research) in the global office equipment market, said the Thai plant was part of plans to spread production geographically.
Ricoh produces more than 70 percent of its copiers and printers in China.
"We hope the Thai plant will grow and become our major production base in the future," Ricoh Executive Vice President Takashi Nakamura told a news conference in Bangkok.
"We are focusing more on Asia and the Thai plant will help add onto our production in China."
Ricoh planned to locate the factory on 11 hectares of industrial land acquired from Amata Corp AMAT.BK, Thailand's largest industrial estate developer. The plant would occupy about half the land, with the rest available for future expansion.
Japanese companies accounted for about 30 percent of the 655 billion baht ($20 billion) in net applications to Thailand's Board of Investment last year.
A survey in late January showed they were slightly more optimistic that business conditions will improve with the country's return to democracy after a 2006 coup.
Shares in Ricoh rose 5.8 percent to 1,684 yen on Thursday after the news. The Nikkei index .N225 gained 4.3 percent.
In Bangkok, Amata shares were up 1.8 percent at 16.90 baht, at 0925 GMT. ($1=108.26 Yen) (Reporting by Noriyuki Hirata, Kiyoshi Takenaka and Viparat Jantraprap; Editing by Darren Schuettler & Jan Dahinten)