There are disclosed a magnet roller which includes a magnet main body portion and a shaft portion, wherein the magnet main body portion includes a composition for magnets and has at least one hollow portion in the inside thereof; a process for producing a magnet roller having a magnet main body portion and a shaft portion, which includes arranging in advance, at least one member for forming a hollow portion in a cavity of a mold which has generated a magnetic field on the circumference of the cavity, pouring a composition for magnets into the mold, and thereafter withdrawing the member for forming the hollow portion so that at least one hollow portion is formed in the inside of the magnet roller; and a developing unit which includes a developing roller constituted of a sleeve installed in a freely rotatable manner and the above magnet roller installed inside the sleeve. The magnet roller can be produced at a low cost owing to curtailed use quantity of a composition for magnets, can suppress the lowering of magnetic force due to voids on the outside periphery thereof, and is favorably employed, for instance, in image forming equipment such as copying machinery, printers and facsimile machinery.