The line was first planned under the
Railway Construction Act in 1922. While this line was not expected to be built as soon as it was planned, the construction to build the line to Asanai Station began during
World War II to transport
fire clay. The first section to Iwate-Wainai opened on June 25, 1942, as Omoto Line. The line was extended to Oshikado on July 20, 1944, although this extension at the time only served freight services. The construction continued after the World War, and the line was finally extended on November 25, 1947, to Utsuno station, located at the exit of the Oshikado Tunnel. Passenger services beyond Iwate-Wainai also commenced with the opening of this extension. After a short pause, the construction was resumed in 1952. The line was extended to Asanai on May 16, 1957, and Utsuno Station was closed on the same date. While the initially planned section from 1922 was completed by this extension, the town of
Iwaizumi was not satisfied with it and began a large-scale movement to extend the line into the town's center. As a result, the extension to
Iwaizumi-Omoto Station was surveyed in 1961. The
Japan Railway Construction Public Corporation began extending the line to
Iwaizumi Station in January 1968, which was completed on February 6, 1972. The line was also renamed to Iwaizumi Line at the same time. While the number of passengers increased upon extension to Iwaizumi, it began to steadily decline from 1975.
Listing as a specified local line The line was listed as one of the
specified local lines, a movement to decommission deficit lines with little passengers in 1982 as they met the criteria for inclusion. Following this event, the
Iwate Prefecture, the municipality of Iwazumi and
Niisato conducted an experiment to see if
JNR bus works as a replacement for the line. The test run on December 24, 1982, found that the
Japan National Route 340 was not wide enough to allow buses to pass each other. Due to this result, the local municipalities argued that bus lines cannot operate to replace the line, although JNR argued that the National Route 340 still works fine enough as an alternative. Local governments, including Iwate Prefecture, raised objection to the decision. In November 2013, JR East announced that agreement had been reached with local governments to formally close the line, which occurred on 1 April 2014. Since 1 April 2014,
Higashinihon Kotsu has operated the
Iwaizumi-Moichi Line which is a
bustitution. ==Services==