A robot cleaner to perform cleaning without interruption of travel when passing over an obstacle. The robot cleaner includes a main body, and drive devices at left and right sides of the main body. Each drive device includes a main wheel to drive the main body, a drive motor to rotate the main wheel, a gear assembly to transmit power from the drive motor to the main wheel, a drive shaft to assist the main wheel in rotating by rotation power, and an auxiliary wheel connected to a distal end of the drive shaft so as to be located closer to the center of the main body than the main wheel. If the main wheel is lifted from a floor when passing over an obstacle during traveling, the auxiliary wheel protrudes downward to reach the floor, assisting the main body in traveling to escape from the obstacle.