A developer supply device is provided, which includes a casing including a developer storage section at a bottom portion therein and an opening formed at an end thereof away from the developer storage section, development agent chargeable with a predetermined polarity, stored in the developer storage section, and a transfer board that is disposed in the casing and configured to transfer the development agent stored in the developer storage section when a multi-phase alternating-current voltage is applied to transfer electrodes of the transfer board. The development agent includes a mother particle having, around an outer surface thereof, an electrically insulating layer without a polar group having a charge polarity identical to the predetermined polarity, and an external additive, absorbed to around the mother particle in an easily desorbable manner, which is an electrically insulating fine particle having a charge polarity identical to the predetermined polarity.