A document reading unit has an image sensor mounted in a box-like support base so that a document can be read by the image sensor through an opening in the support base. A contact glass disposed in the opening covers the image sensor and supports the document while it is being read by the image sensor. Support members coupled to the contact glass are detachably fixed to the support base by fixing screws. The image sensor has two rows of spaced-apart line sensors with the line sensors in one row staggered relative to those in the other row, and the support members are arranged between adjacent pairs of line sensors in each row of line sensors. The contact glass and support members are removable as a unit from the support base.