A focus adjusting device (50) for an image sensor (1) is provided for ensuring precise focusing of the image sensor. The focus adjusting device includes a contact element (51) in direct contact with the bottom of a scanner cover plate, a focus varying element (53) engaging with the contact element, and a focus fine adjusting element (55) movably mounted on a housing of the image sensor. The focus varying element and the contact element are driven to correspondingly move vertically together with the vertical movement of the focus fine adjusting element, whereby the distance between an upper surface of the housing and the bottom of the cover plate is thus changed for precise focusing.