In an image forming apparatus, a case containing a developer includes a first storage storing developer information indicating a composition of the developer. A developing part receives the developer from the developer-containing case. An acquiring part acquires the developer information in the first storage, when the developer-containing case is installed to an installed part. A decision part decides whether or not a composition of the developer in existing developer information in a second storage is different from another composition of the developer in newly acquired developer information. An electric charge amount measuring part measures an electric charge amount of the developer in the developing part. A developer replacement controlling part controls discharge and replenishment of the developer in the developing part on the basis of the measured electric charge amount, when the decision part decides that both compositions are different.