In certain range of Reynolds number and rotational speed, spheres and cylinders rotating about a cross-flow axis in a uniform stream suffer the lifts which have direction opposite to those in ordinary Magnus effect. In the present experiment the condition under which the negative Magnus force takes place was clarified for the case of a sphere. And it was found that this remarkable effect can be explained as a transition effect of the boundary layer from laminar to turbulent flow. The photographs of the actual flow pattern around a rotating sphere was also taken by means of aluminium dust method, and the difference between the two flow-patterns of ordinary Magnus effect and negative Magnus effect was revealed. 1. Introduction and Outline of the Experiments / 2. Apparatus / 3, Results and Discussion