Abstract: A magnetic brush development device has a development casing housing a development sleeve for carrying a magnetic brush thereon, the development sleeve confronting an electrostatic latent image carrier such as a photosensitive drum. When the development sleeve rotates, toner carried by the development sleeve acts as a pump for air inside the development casing housing, and the pumping action of the air causes developer to be scattered out of the housing. However, according to the invention, the development casing has spaces defined therein around the development sleeve and communicating with each other to equalize pressures therein for thereby preventing developer from being scattered out.