A scanning apparatus for preventing defocus aberration is provided. The scanning apparatus includes a flatbed scanning portion and a scanning module. The flatbed scanning portion includes a glass platform. The scanning module includes a scanning module case, a light source, multiple reflective mirrors, a lens, an optical sensing element, a printed circuit board and a metallic post. The metallic post is interconnected between the scanning module case and the printed circuit board. The printed circuit board is not in direct contact with the scanning module case so as to prevent defocus aberration resulting from thermal expansion.