This phantom object is designed for a Winston-Lutz test on a device for radiation therapy treatment comprising a patient support having a rotation axis (V), a stand having a rotation axis (H) and a collimator having a rotation axis (C), locating means being provided for visually representing the theoretical positions (V′, H′, C′) of the three axes (V, H, C). A spherical ball is positioned at the center of a sphere in a material of electron density lower than that of the ball, the ball and the sphere both constituting the phantom object. The sphere has on its outer surface means for a visual alignment with the locating means allowing a positioning of the phantom object at the point of convergence of the three axes (V′, H′, C′).