<p>A developing device, a process cartridge and an image forming device having their image densities sustained even if the outer diameter of a developing agent carrier is up to 12 mm, and fogging or density non-uniformity controlled to up to an allowable level. The outer diameter of a developing agent carrier (41) is at least 8 mm and up to 12 mm; and a magnetic one-component developing agent (43) has saturated magnetization of at least 20 Am<SUP>2</SUP>/kg and up to 37 Am<SUP>2</SUP>/kg when a magnetic field of 79.6 kA/m (1000 oersteds) is applied, magnetization of at least 70% and up to 80% of saturated magnetization when a magnetic field is reduced to 55.7 kA/m (700 oersteds), and magnetization of at least 50% and up to 62% of saturated magnetization when a magnetic field is reduced to 39.8 kA/m (500 oersteds).</p>
