an image capturing device and its assembly method are disclosed . the image capturing device comprises a main board , a metal plate and a sensing unit . the main board comprises a first hollowed block . the metal plate comprises a second hollowed block corresponded to the first hollowed block and is disposed at one side of the main board . the sensing unit is disposed at the other side of the main board . the metal plate is parallel to the sensing element . the first hollowed block and the second hollowed block are filled by a fastening material . in addition , the image plane of the sensing unit is perpendicular to the optical axis of the lens set . therefore , image quality is improved .