Patent ID: 8515296

Claim:
A developer supplying device comprising: a supplementary developer container configured to contain a supplementary developer including a toner and a carrier mixed in a predetermined ratio, wherein the supplementary developer container is replaceable; a supplementary developer feeding passage configured to feed the supplementary developer from the supplementary developer container to a developing device; a carrier concentration determining device, which is provided on the supplementary developer feeding passage to determine concentration of the carrier in the supplementary developer fed through the supplementary developer feeding passage; and a residual supplementary developer amount estimating device configured to estimate an amount of the supplementary developer remaining in the supplementary developer container on the basis of the carrier concentration, wherein the residual supplementary developer amount estimating device determines at least one of an end state and a near-end state of the supplementary developer in the supplementary developer container when the carrier concentration determined by the carrier concentration determining device reaches a predetermined first carrier concentration and then reaches a predetermined second carrier concentration lower than the predetermined first carrier concentration.