The present invention is directed to an assembly of an optical fiber and an optical fiber holder for holding the optical fiber, the optical fiber having an end surface formed at an end portion thereof, the end surface being configured to perform light coupling with a light emitting element or with a light receiving element. The semiconductor package according to the present invention enables mass production at wafer level, is easily stacked between the packages, and has an excellent electrical characteristic. The lower rail is constituted by one narrow plate rail having a lower end edge being a free end, the lower guide rollers include a set of two or more rolling rollers, and the set of two or more rolling rollers hold and support the lower rail therebetween from below so as to freely roll.