An image forming apparatus includes a rotatable drum, a charging section for charging the drum, a latent image forming section, a potential detector, a phase detector, and a controller. The charging section includes a first carrying member and a second carrying member for carrying electroconductive magnetic particles. The second carrying member is disposed downstream of said first carrying member. The latent image forming section forms a latent image on the drum, and is disposed downstream of the charging section. The potential detector detects a surface potential of the drum, and is disposed downstream of the charging section. The phase detector detects information relating to a rotational phase of the second carrying member. The controller controls voltages applied to the first carrying member and to the second carrying member, on the basis of a result of detection by the potential detector and a result of detection by the phase detector.