Opened in 1932 together with the Kamimiyori ~ Yunokami section of the Aizu Line. This was the 400m-long tunnel into which the line went (upwards) immediately off the north end of the #2 Ogawa Bridge. After exiting the No. 4 tunnel, the line ran through cedar forest.
The construction method was the same as for the rest of the Onumazaki tunnels, with the original tunnel wall being made entirely of concrete bricks, with a lining of poured concrete up until about halfway up the side walls. Later, the poured concrete layer on the lower half of the walls was given an extra layer, and a layer of wooden boards was applied to the arch of the ceiling where the brick was exposed; this was fastened in place by iron ribs that ran all the way around the arch from floor to floor.
トンネルは１９３２年で上三寄駅から湯野上駅までの路線部分と一緒に開かれた。上りへ行って、この長さ４００mのトンネルは第二大川橋梁の北端の直後であった。トンネルから出て線路は杉の森の中走った。
残りの小沼崎トンネルたちのようにトンネルの元の壁はコンクリートレンガで作られた、壁の下半分にて注いたコンクリートの裏地があった。後ほど、コンクリートの裏地で追加の地層付け加えられた、と丸出しレンガは木の板で覆われていた。木の板は鉄の肋骨で開催された。
