This paper presents a numerical investigation of a disk-shaped rotor rotating inside a granular medium using the discrete element method, and the torque, quantitatively characterizing the resultant of drag forces on the rotor from surrounding particles, is mainly explored. The existence of a dumbbell-shaped region induced by the rotating rotor may essentially give rise to the steady state independent of initial loading conditions, in which the pressure difference between the upper and lower surfaces of the disk disappears and the torque always tends to a stationary value. Furthermore, the rotation of the rotor induces an anisotropic structure with the formation of arches that screen the vertical pressure, leading to the occurrence of the low-pressure region above the rotor. Finally, a scaling law of the torque with the depth is proposed, in which the rotation speed of the rotor as an affecting factor is also taken into account. (C) 2020 Elsevier B.V. All rights reserved.