An undercutting tool for producing transverse grooves in a drilled hole. An upper support and a lower support axially delimit a receiving region of a shank. At least two spreading elements are disposed in the receiving region between the spreading elements and the receiving region. The shank and the supports are disposed so the axial distance between the supports can be reduced by exerting an axial force. The spreading elements each have an oblique upper and lower end face, which each have such an outer geometric shape, extending in a direction not perpendicular to the longitudinal axis and are displaced in substantially opposite radial directions, and the axial force is in each case converted to a radial force in the radial directions.