A developing device includes a developer carrier, a developing unit housing which rotatably supports the developer carrier, a side seal member which comes into slidable contact with both ends of the developer carrier, a sheet-like elongated seal member which extends in the axial direction of the developer carrier so as to come into slidable contact with the developer carrier, a side seal attachment surface which is formed in the developing unit housing and to which the side seal member is attached, and a support portion which is formed in the developing unit housing and protrudes toward the developer carrier from the side seal attachment surface to support the elongated seal member. The elongated seal member is attached onto the support portion in a state where both ends thereof overlap the side seal member and are opposite the side seal attachment surface.