An image forming apparatus executes a first obtain mode to obtain a resistance value of a developing roller when a potential difference between the developing roller and an electroconductive roller is generated by a second voltage generator when the developing roller carries a reference liquid developer having a known toner concentration. A second obtain mode is executed to obtain a toner concentration of the liquid developer on a surface of the developing roller based on the obtained resistance value and a detection result when a potential difference between the developing roller and the electroconductive roller is generated by the second voltage generator when the liquid developer is applied to the developing roller. During image formation, the controller controls a first voltage generator based on the obtained toner concentration so the toner concentration of the liquid developer on the surface of the developing roller is a target value.