From Game Developer Research Institute
Ikegami Tsushinki Co., Ltd. (池上通信機), founded on September 10, 1946, and incorporated on February 21, 1948, is a manufacturer of broadcasting equipment.  In the late 1970s and early 1980s, the company was involved with developing arcade games, primarily for Nintendo.
Ikegami's relationship with Nintendo began when Ikegami took a request from Tokuzo Komai of Nintendo Leisure System to develop and manufacture arcade games exclusively for Nintendo, which would sell them as their own product. The contract included eight titles.
Among those games was Radar Scope, which Ikegami designed and developed for Nintendo. The game was popular briefly in Japan, prompting Nintendo of America President Minoru Arakawa to place a large order for it. But by the time units reached the States, any buzz surrounding the game had dissipated, and arcade operators were left unimpressed. Facing financial disaster, Arakawa asked his father-in-law and Nintendo CEO Hiroshi Yamauchi to provide him with a game that could be installed as a replacement. 
Yamauchi assigned a young Shigeru Miyamoto with designing a new game using the Radar Scope hardware. What Miyamoto came up with was Donkey Kong. Nintendo decided to work with Ikegami, which had the technology to program it. Ikegami wrote all the code and sold Nintendo 8,000-20,000 PCBs (Nintendo copied 80,000 without permission). Since there was no contract between Ikegami and Nintendo, Nintendo did not have the source code. But Nintendo wanted a sequel, so DK was disassembled and reverse engineered (through subcontractor Iwasaki Giken) and soon came Donkey Kong Junior (noted as being the first Nintendo game developed entirely in-house). In 1983, an angered Ikegami sued Nintendo for ¥580,000,000 for copyright infringement, claiming it owned the original DK code. In 1990, the two companies settled out of court. The details of that were not released to the public. In another trial that year, it was determined that Nintendo did not hold the copyright to the DK code.
If you look at the tilesets from Congo Bongo, Donkey Kong, and Zaxxon, you will find the then Ikegami logo  (see right; below that: the logo as seen on a 1978 Ikegami color Handy Looky viewfinder). It also turns up in Donkey Kong Junior and Super Zaxxon, but Ikegami's involvement with the latter is unknown (the situation with DK Junior was explained earlier; Super Zaxxon is essentially a more challenging version of Zaxxon). It is also unknown if Ikegami was involved with Future Spy, a conversion for Zaxxon.
The following text appears in one of the ROMs from the Japanese version of Donkey Kong:
CONGRATULATION !IF YOU ANALYSE DIFFICULT THIS PROGRAM,WE WOULD TEACH YOU.*****TEL.TOKYO-JAPAN 044(244)2151 EXTENTION 304 SYSTEM DESIGN IKEGAMI CO. LIM.
A number of other Nintendo arcade games besides the ones listed below have been said to be by Ikegami including Block Fever, Popeye, Sheriff, Space Fever, Space Firebird, Sky Skipper, Computer Othello, Space Launcher, and Heli Fire.  However, there does not seem to be any concrete proof that any of those were done there.
- Donkey Kong (Nintendo)
- Radar Scope (Nintendo)
- Tip Top / Congo Bongo (Sega)
- Zaxxon (Sega)
- 1. Sore ha Pong kara Hajimatta: Arcade TV Game no Naritachi, Masumi Akagi, Amusement Tsuushinsha, 2005.9, ISBN 4990251202.