A sheet feeding device for use in an image forming apparatus provided with a sheet support plate and one or more rotary members for separating and feeding sheets. A recessed portion is formed on the sheet support plate in a position opposing to each rotary member. A detachable auxiliary member is provided to be fitted in the recess to form a plane coplanar with that of the sheet support plate. An engaging device can be provided, to engage with a step portion formed in a cassette housing portion of the apparatus to keep the cassette from being entirely removed from the housing. The engaging device may, for example, include an elastic part and an engaging part integrally molded with each other.