Multicopters for facility inspection and logistics applications are often required to fly very close to structures. However, it is known that the behavior of multicopters can be unstable in the vicinity of structures. In this study, the ground effect when there is an inclined ground plane below the quadcopter was investigated experimentally. The results show that imbalanced aerodynamic forces that can disturb the flight are generated by different mechanisms depending on the propeller configuration and flight altitude: when flying near an inclined plane with the thrust force of the four rotors maintained constant, the torque acting on the rotors changes depending on the inclination angle of the ground plane. The change in torque is influenced by the distance between the rotor and inclined plane and by the distance between neighboring rotors. It is suggested that an imbalance of yawing moments may occur in the quadcopter, which makes flight unstable, depending on the azimuth angle of the quadcopter with respect to the inclined plane. This study also examined the ground effect of ducted rotors above the inclined ground plane and observed different characteristics from the case without ducts.