the rotational driving force of the drive motor 60 is adapted to be transmitted to the lower shafts 54a 54b respectively in the first embodiment whereas the rotational driving force of the motor 60 is adapted to be transmitted to the main shafts 49a 49b respectively in the second embodiment in fig1 a large diameter shaft 74 is supported by bearings 73 fixed on the front surface of an upper frame 42 between the inner ends of the main shafts 49a 49b which locate at the center of the sewing machine and the inner ends of the main shafts 49a 49b are connected to the large diameter shaft 74 via couplings 75 respectively a timing pulley 76 is fitted to the center of the large diameter shaft 74 and another timing pulley 62 is fitted on a motor shaft 61 of the drive motor 60 fixed to the rear surface of the upper frame 42 see fig1 a timing belt 63 penetrating the upper frame 42 through windows 77 see fig5 defined in the front wall and the rear wall of the frame 42 respectively is wrapped around the timing pulley 62 and the timing pulley 76 of the large diameter shaft 74 the lower shafts 54a54b are connected to each other via a coupling 78 in this second embodiment when the drive motor 60 is driven to rotate in a certain direction the main shafts 49a 49b are rotated via the center timing belt 63 since the lower shafts 54a54b are also rotated via the side timing belts 68 the needle bars 12 the thread take up levers 13 and the shuttles.