The invention relates to an object localization apparatus for localizing an object having markers based on a projection image. An object function (8) is provided along x-z positions (9, 20 . . . 25), wherein the x-z positions (9, 20 . . . 25) are defined by pairs of an x position being a position of a recognized projected marker along a fictive line on the recognized projected object in a projection plane and a z position defining a position in a direction being outside the projection plane. The position is determined based on the object function (8) which is modified by varying the z positions such that distances between the x-z positions along the object function (8) are adapted to distances between the markers of the object. This allows the object localization apparatus to determine the position with a reduced number of projection images, in particular, with only a single projection image.