A flat cable winding device includes a case, a flat cable guide portion provided in the case and that introduces the flat cables into the case separately, a rotary table that is rotatable around a central shaft portion, a biasing unit that rotates and urges the rotary table in a winding direction of flat cables, a plurality of rollers that are rotatable around the central shaft portion on the rotary table, a separation guide path that guides the flat cables introduced into the case from the flat cable guide portion to the winding start position separately, and a slack absorption unit provided in the separation guide path and that absorbs slack of a flat cable located at an inner peripheral side.