An automatic document feeder includes a transfer channel, a pick-up module and a thickness detecting module. The thickness detecting module includes a detecting member, a transmission member and an optical displacement sensing member. By the pick-up module, a sheet article is fed into the transfer channel. When the sheet article is transported across the detecting member, the sheet article is sustained against the detecting member, so that the detecting member is moved. The transmission member is connected to the detecting member. As the detecting member is moved, the transmission member generates a displacement amount. The optical displacement sensing member is used for detecting a displacement amount of the transmission member, thereby acquiring the thickness of the sheet article.