Image forming apparatus

An image forming apparatus comprising:

• an image carrying portion including:
• an image carrying member having a photosensitive layer formed on a surface thereof;
• a charging device that electrostatically charges the image carrying member;
• an exposure device that forms an electrostatic latent image by shining light to the image carrying member electrostatically charged by the charging device;
• and
• a developing device having a developer carrying member that is disposed opposite the image carrying member and that carries two-component developer containing magnetic carrier and toner, the developing device forming a toner image by attaching the toner to the electrostatic latent image formed on the image carrying member,
• a developing voltage power supply that applies to the developer carrying member a developing voltage having an alternating-current voltage overlaid on a direct-current voltage;
• a density sensing device that senses a density of the toner image formed by the developing device;
• a current sensing portion that senses a direct-current value of a direct current passing between the developer carrying member and the image carrying member;
• and
• a control portion that controls the image carrying portion and the developing voltage power supply,
• wherein
• the control portion can perform at least one of
• a first potential difference adjustment mode in which the control portion sets, based on a predicted fogging toner amount, a blank-part potential difference between an unexposed-part surface potential on the image carrying member and a direct-current component of the developing voltage during image formation and
• a second potential difference adjustment mode in which the control portion sets, based on a predicted carrier development amount, the blank-part potential difference during image formation,
• in the first potential difference adjustment mode, the fogging toner amount is predicted from a toner charge amount and a fogging toner current sensed by the current sensing portion with a zero surface potential on the image carrying member, and
• in the second potential difference adjustment mode, the carrier development amount is predicted based on a toner concentration, a carrier charge amount calculated based on the toner charge amount, and a carrier development current sensed by the current sensing portion with a zero surface potential on the image carrying member.