To provide a developing device or the like, capable of surely preventing the leakage of developer without being influenced by the variations in the applied amount or the like of a sealing member.

The developing device is equipped with a developing frame body 45 for holding a developing roller for developing an electrostatic latent image formed on a photoreceptor drum; a developer-storing frame body 41, having a developer storing part in which the developer supplied to the developing roller is stored and coupled with the developing frame body 45; a recessed part 454 or 455, which is provided at the coupling part of the frame body of either the developing frame body 45 or the developer-storing frame body 41, and to which the sealing member 100 or 101 is applied; and a compression rib 414 or 415, provided at a position where it is opposed to the recessed part 454 or 455 at the coupling part of the frame body of the other of the developing frame body 45 and the developer storing frame body 41. When the sealing members 100 and 101 are applied to the recessed parts 454 and 455 so as to couple the developing frame body 45 with the developer storing frame body 41, the compression ribs 414 and 415 are made to advance into the recessed parts 454 and 455.