The line opened on 2 January 1945 as the freight-only operating between and (present-day
Naganohara-Kusatsuguchi Station). Passenger services were introduced as far as from 5 August 1945, to from 20 November 1945, and to Naganohara-Kusatsuguchi from 20 April 1946. On 1 October 1952, the line was extended to , initially for freight only, but passengers services were also introduced from 21 June 1954. Freight services between Shibukawa and Ōshi ceased as of 1 October 1966. Services on the section between Naganohara and Ōshi were suspended as of 1 November 1970, and on 7 March 1971, a new line was opened beyond Naganohara to , with the entire line renamed Agatsuma Line at the same time. The Naganohara-Kusatsuguchi to Ōshi line was formally closed as of 1 May 1971. CTC signalling on the entire line was also commissioned at the same time. With the privatization of
JNR on 1 April 1987, the Agatsuma Line came under the ownership of
JR East. It had been proposed to extend the line to Nagano, but geological exploratory drilling revealed extensive faulting beyond Ōmae, and as any extension would have required extensive tunnelling, this was considered impractical. ==Yamba Dam construction==