An image forming apparatus, including a photosensitive member, a charger configured to charge the photosensitive member, a developer roller configured to contact the photosensitive member and supply a developer agent to the photosensitive member, a humidity sensor configured to detect humidity, and a controller is provided. The controller is configured to a voltage-application controlling process, in which the controller controls the photosensitive member and the developer roller to rotate, applies a charger-voltage to the charger, and applies a developer-voltage to the developer roller. In the voltage-application controlling process, the controller controls a voltage difference between the charger-voltage and the developer-voltage based on a peripheral velocity ratio of the developer roller with respect to the photosensitive member and the humidity.