A step-by-step motor able to carry out up-and-down motion includes a housing, a connecting base, a top rod, a bearing, a connecting member, a magnet, a right-handed coil, a reverse coil and a bottom cover combined together. The housing has one side bored with an opening and its inner wall bored with a slide groove, and the connecting base has one outer wall formed with a slide rail for combining the connecting base on the outer wall of the housing. The bottom cover is provided with an annular recess for engaging the bottom edge of the housing, and then the bottom cover and the housing are firmly combined together by supersonic welding. By so designing, the step-by-step motor can be assembled conveniently, quickly and stably.