Provided is a core/sheath composite filament for toothbrushes which exhibits excellent hardness, durability and feeling of use when used in toothbrushes, is less prone to falling out when tufted into the tufting plate, and makes it possible to improve stain-removal properties. Also provided is a toothbrush using said core/sheath composite filament. A toothbrush is configured using bundles of bristles configured from core/sheath composite filaments that have a quadrangular cross section in the plane perpendicular to the lengthwise direction and comprise a core (2), which is formed from a synthetic resin fiber, and a sheath (3), which integrally covers the core (2) and is formed from an elastomer that is compatible with the synthetic resin constituting the core (2). Tufts configured from a plurality of the core/sheath composite filaments (1) are tufted in the center portion of a tufting plate.