An image forming apparatus includes a latent image carrier; a charger; a charging power supply; a latent image writer to write; a developing device; a rotary attitude sensor; a controller to perform a data formulating process to formulate a change pattern data and a bias changing process to cause the charging power supply to change the charging bias output based on readings from the rotary attitude sensor and the change pattern data, during a print job; and a surface potential sensor to detect a surface potential of the latent image carrier. In the data formulating process, the controller formulates the change pattern data based on the surface potential of the latent image carrier detected by the surface potential sensor, while the charging power supply outputting a constant charging bias, and based on readings from the rotary attitude sensor while the surface potential is being detected.