It is generally difficult to measure the particle motion inside of dense granular flows or fluid-particle flows. In the present paper, an MRI (Magnetic Resonance Imaging) technique is employed to measure the granular motion inside a rotating horizontal drum. A horizontal drum is half-filled with a mixture of two different sizes of spherical particles, and is steadily rotated along its axis. A typical radial segregation phenomenon, i.e., smaller particles concentrate to the central part and larger particles distribute in the peripheral part, was observed in vertical cross sections. When a tapered drum was used, in addition to the radial segregation, an axial segregation was also observed. The effect of the inclination angle of the drum wall on the segregation phenomena was studied. Furthermore the effect of the rotational speed on the axial segregation phenomena was also studied.