A mount-demount mechanism includes a unit that is mountable on and demountable from an apparatus main body, and a pair of slide rails that are disposed on the apparatus main body and slidably support both surfaces parallel to a mount-demount direction of the unit. Provided, on both side surfaces thereof parallel to a mount-demount direction of the unit, are a pair of unit-side rollers that include a first roller that is disposed in a downstream insertion direction and a second roller that is disposed in an upstream insertion direction. The slide rail is provided with a rail groove that includes a lower rail and an upper rail, a cut-away portion for allowing the unit-side roller run off from the rail groove is formed on a portion of the upper rail or the lower rail, wherein either of the first roller and the second roller runs off from the rail groove.