A developing device that can be formed at low cost and without increasing the complexity of a die structure of a casing comprises a development roller, a supply roller, a developing portion having an agitating member, a toner hopper disposed above the developing portion, an agitating member in the toner hopper, a partitioning member provided between the developing portion and the toner hopper and having an opening portion, and a collected developer storage portion forming portion for forming a storage space for collected toner collected from the development roller by the supply roller within a fixed distance of the supply roller such that the collected toner moves to the vicinity of the agitating member in the developing portion in accordance with a developer flow generated by the rotation of the supply roller. A pre-incorporation component formed by integrating the partitioning member, the collected developer storage portion forming portion, and a bearing portion for supporting the agitating members is incorporated into the casing.