In 2005, entrepreneurs on the island of Shikoku created an independent professional minor league designed to appeal to local baseball fans. Shikoku is not home to any of the Nippon Professional Baseball (NPB) teams. The two main sponsors of the initiative were JR Shikoku and Shikoku's Coca-Cola Bottling Company. (Japanese railway companies public and private have long been major sponsors of professional baseball teams.) Among the strategies for building a local fan base are uniquely localized names and the hiring of Shikoku natives for fill-in roles like designated hitters, pinch hitters, and such. Like NPB's Pacific League, the Shikoku Island League employs DHs. The team names are just as quirky and unique as those of North American minor league teams like the Albuquerque Isotopes, Lansing Lugnuts, Montgomery Biscuits, or Savannah Sand Gnats. All the team names are written in katakana but abbreviated in roman capital letters. The Island League (IL) logo and mascot is a blue and white Manta Ray, for its baseball-diamond shape.
Tokushima IndigoSocks (IS) - Tokushima Prefecture is famous for its indigo, so it's not surprising that the team color is blue. The mascot is a spider, who wears four pairs of socks. The IndigoSocks won the 2019 league championship, but lost to the Tochigi Golden Braves in the interleague championship.
Kagawa Olive Guyners (OG) - Takamatsu in Kagawa Prefecture is the league headquarters, and the Olive Guyners have won the most league championships so far. Kagawa is famous in Japan for its olives and olive oils, home games are played in Olive Stadium, and the team color is green. Guyners is an anglicized rendering of the local Sanuki dialect word gaina 'strong'.
Kochi Fighting Dogs (FD) - Kochi Prefecture (once known as Tosa Domain) is famous for its Tosa fighting dogs, Japanese mastiffs, so its team name and mascot were easy to choose. The team color is black and their gray mascot wears a yokozuna belt like that of sumo champions. The FD won the first league championship in 2005, but haven't done so well since then. In 2017 they hired Manny Ramirez but he left in mid-August with a knee injury.
Ehime Mandarin Pirates (MP) - Ehime Prefecture is famous for its mandarin oranges (mikan) and its seafaring heritage. Their basketball team is the Ehime Orange Vikings. The team color is orange in both cases.
In 2007, the league expanded to include two teams on Kyushu and changed its name to the Shikoku-Kyushu Island League. But the Nagasaki Saints (named for the prefecture's long Roman Catholic heritage) and Fukuoka Red Warblers (named for the color of ume and the Japanese bush warbler) didn't last long. Nor did the Three Arrows team from Mie (三重 'three weights') Prefecture, on Honshu across the Kii Channel from Tokushima. So now the official name of the league is Shikoku Island League plus, presumably to allow for other expansion attempts.
In 2014, two independent baseball leagues, Shikoku Island League plus and Route Inn Baseball Challenge League, formed the Japan Independent Baseball League Organization. The champions of each league play each other at the end of each season. Shikoku Island League plus has also sent all-star teams to play all-stars from the independent Can-Am League in North America.