This invention provides a developing device, a process cartridge and an image forming apparatus which can suppress and prevent the slipping-through of a developer to the end portions of a developing sleeve and can suppress and prevent the leakage of the developer by a shock or the like. A developer sleeve 31 having a magnet roller 32 therein is disposed in the opening portion of a developing container 30, and magnet seal members 34 for regulating the movement of the toner are disposed in non-contact with the developing sleeve 31 on the outer peripheral surface of the opposite ends of he developing sleeve 31 which is adjacent to the developing container 30. The magnet roller 32 has a plurality of magnetic poles, and N and S poles are magnetized to multiple magnetic poles on the inner peripheral surfaces of the magnet seal members 34, and the magnet roller and the magnet seal members are formed so that the peak value Br1 of the magnetic-flux density by the magnetic poles of the magnet roller in the direction of a normal to the surface position of the developing sleeve may be smaller than the peak value Br2 of the magnetic-flux density by the magnetic poles of the magnet seal members in the direction of the normal to the surface position of the developing sleeve.