A casting die assembly which does not produce any burrs includes a stationary die and a movable die that are clamped in a condition in which a slide core is caused to move toward a center of a movable die, and a back surface of a protrusion of the slide core contacts a stopper section provided on a frame section. The stopper section is subjected to the pressure applied to the slide core during a casting operation. Since the stopper section is provided on part of the frame section, which is integrally formed on the periphery of the stationary die, the stopper section is not caused to retreat or deform by the casting pressure and as a result, the slide core is not moved and burrs are not produced.