A reading unit provided with an optical system mounted on a recording apparatus to read a source document includes a housing having the optical system in it, a light transmitting portion arranged on the outer surface of the housing of the reading unit in order to guide the light outside the reading unit into the optical system of the reading unit, and a protection member to cover the outer side of the light transmitting portion for opening and closing such portion freely. With the structure thus arranged, the reading unit is protected from ink mist and other particles created by the recording apparatus that may contaminate the reading unit, hence preventing the accuracy of reading signals from being lowered when reading is performed by means of this reading unit.