Abstract: Provided is a binder fiber for an optical fiber unit including a flat sea-island color composite fiber, in which the flat sea-island color composite fiber satisfies the following (1) to (3): (1) a sea component of the flat sea-island color composite fiber has a melting start temperature of 100° C. or higher and a melting peak temperature of 120° C. to 150° C.; (2) the flat sea-island color composite fiber has a width of 0.5 to 3.0 mm and a thickness of 0.15 mm or less; and (3) the flat sea-island color composite fiber has a thermal shrinkage rate of 1.0% or lower after being heated at 100° C. for 3 hours. With the binder fiber, the color developing properties are improved, the shape retaining properties of optical fiber cores are maintained, the optical fiber cores are not compressed, and there is no adhesion with another adjacent binder fiber or the optical fiber cores.