A hinge mechanism of the present disclosure includes a pair of hinge units each having a hinge base, a hinge upper member, and a shaft, and the hinge mechanism holds an automatic document feeder to be openable/closable in up/down directions with respect to an image reading unit. The hinge base is supported on the image reading unit which reads a document image. The hinge upper member is supported on the automatic document feeder which feeds a document to the image reading unit. The shaft pivotably couples the hinge base and the hinge upper member together. By attaching the hinge upper member to the automatic document feeder, a groove formed in a circumferential surface of the shaft in an annular shape in a circumferential direction of the shaft and a rib provided on the automatic document feeder engage with each other, whereby axial movement of the shaft is restricted.