A reader includes a housing with a platen on which a document is placed, a light source unit which moves along the platen and irradiates light toward the document, a mirror unit which works with the light source unit to define an optical path of the reflected light from the document in the housing, and a rotatable lock member mounted on one of the light source and mirror units. The housing includes an operation member, which is slid to fix the light source and mirror units, and a slide surface, which holds the sliding operation member. The lock member is rotated between an engaged position, where the lock member is engaged with another of the light source and mirror units, and a disengaged position, where the lock member is disengaged from the other of the light source and mirror units in response to slide operation of the operation member.