An image reading apparatus having a first function of reading a document being conveyed and a second function of reading a document placed on a placing surface, the image reading apparatus comprising: an imaging unit provided below the placing surface and configured to be moved along the placing surface; a moving mechanism configured to move the imaging unit between a first position at one end side in a moving direction of the imaging unit and a second position at another end side in the moving direction of the imaging unit; a conveyance mechanism configured to convey a document upon operation of the first function; a sun gear configured to be rotated by a driving force applied from a driving source in a forward direction or a reverse direction; a planetary gear configured to rotate by being meshed with the sun gear and having a rotation center configured to revolve around the sun gear between a third position and a fourth position, a rotation center of the sun gear serving as a revolution center; a first output gear configured to be meshed with the planetary gear and transmit a driving force to the moving mechanism, when the planetary gear is located at the third position; a second output gear configured to be meshed with the planetary gear and transmit a driving force to the conveyance mechanism, when the planetary gear is located at the fourth position, characterized by a load generation part configured to increase a rotational resistance of the first output gear when the imaging unit is located at the first position, compared to the rotational resistance of the first output gear when the imaging unit is located at a position other than the first position, and a meshing part configured to be meshed with teeth of the planetary gear when the planetary gear revolves around the sun gear between the third position and the fourth position. an imaging unit provided below the placing surface and configured to be moved along the placing surface; a moving mechanism configured to move the imaging unit between a first position at one end side in a moving direction of the imaging unit and a second position at another end side in the moving direction of the imaging unit; a conveyance mechanism configured to convey a document upon operation of the first function; a sun gear configured to be rotated by a driving force applied from a driving source in a forward direction or a reverse direction; a planetary gear configured to rotate by being meshed with the sun gear and having a rotation center configured to revolve around the sun gear between a third position and a fourth position, a rotation center of the sun gear serving as a revolution center; a first output gear configured to be meshed with the planetary gear and transmit a driving force to the moving mechanism, when the planetary gear is located at the third position; a second output gear configured to be meshed with the planetary gear and transmit a driving force to the conveyance mechanism, when the planetary gear is located at the fourth position, characterized by a load generation part configured to increase a rotational resistance of the first output gear when the imaging unit is located at the first position, compared to the rotational resistance of the first output gear when the imaging unit is located at a position other than the first position, and a meshing part configured to be meshed with teeth of the planetary gear when the planetary gear revolves around the sun gear between the third position and the fourth position.