An object of the present invention is to facilitate the inspection of a driving unit for a lifting mechanism for lifting up sheets stacked in a sheet cassette and the detachment thereof from a main body of an image forming apparatus. The stress plate consists of a pre-fabricated metal layer and a thermoplastic layer which are integral with each other. To achieve a high-quality, hot-rolled metal strip with flatness tolerances comparable to those which can currently be achieved in the production of hot-rolled metal strip from continuous-cast thin slabs or slabs, at cast thicknesses of between 40 and 300 mm, in a continuous production process starting directly from metal melt. The extension portion includes a projecting portion which protrudes radially and outwardly of the extension portion at or adjacent to a proximal end of the extension portion such that a seal groove is formed between the projecting portion and the joint body, and a nail portion which protrudes radially and outwardly of the extension portion at or adjacent to a distal end of the extension portion. The electrical circuit is enclosed in the housing and a built-in-test circuit is electrically connected to the electrical circuit .