A binder fiber for an optical fiber unit, the binder fiber comprising: a flat sea-island color composite fiber that includes a sea component and an island component, wherein the flat sea-island color composite fiber is composed of a bundle of a plurality of core-sheath color composite spun fibers, which are comprised of thermoplastic resins, the thermoplastic resins includes a sheath-component resin and a core-component resin, the core component resin having a melting point which is higher than a melting point of the sheath-component resin by 20°C or more, the sea component is formed of fused and integrated sheath-component resin of the bundle, suitable to have been made by drawing the bundle at a temperature which is the melting point of the sheath component resin or higher and lower than the melting point of the core component resin, the island component is composed of fibers formed of the core-component resin, the fibers being dispersed in the sea component in an island shape, and the flat sea-island color composite fiber satisfies the following to: the 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; the flat sea-island color composite fiber has a width of 0.5 mm to 3.0 mm and a thickness of 0.15 mm or less; and 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. a flat sea-island color composite fiber that includes a sea component and an island component, wherein the flat sea-island color composite fiber is composed of a bundle of a plurality of core-sheath color composite spun fibers, which are comprised of thermoplastic resins, the thermoplastic resins includes a sheath-component resin and a core-component resin, the core component resin having a melting point which is higher than a melting point of the sheath-component resin by 20°C or more, the sea component is formed of fused and integrated sheath-component resin of the bundle, suitable to have been made by drawing the bundle at a temperature which is the melting point of the sheath component resin or higher and lower than the melting point of the core component resin, the island component is composed of fibers formed of the core-component resin, the fibers being dispersed in the sea component in an island shape, and the flat sea-island color composite fiber satisfies the following to: the 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; the flat sea-island color composite fiber has a width of 0.5 mm to 3.0 mm and a thickness of 0.15 mm or less; and 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. the 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; the flat sea-island color composite fiber has a width of 0.5 mm to 3.0 mm and a thickness of 0.15 mm or less; and 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.