For an image-capturing unit having a light source and mirrors directing light reflected from a scanned document into a lens condensing the light onto a photoelectrically converting image sensor, a synthetic resin housing. The housing is composed of an open-ended upper frame having a top plate from which sidewalls extend, encompassing an internal space. A mounting surface for anchoring the light-source is formed on the upper side of the upper-frame top plate, and formed on the lower side of the upper-frame top plate is a pedestal on which the lens is supported. Mirror-support projections are formed on the lower side of the upper-frame top plate, or on the upper-frame sidewalls, to support the mirrors from below the lens. With this housing, the influence of heat from the light source on the optics is minimized, improving the mirrors' positional accuracy.