25 of the ac servo motor 1 is coaxially connected to the main shaft 12 by the coupling 29 a loss of transmission of the driving force of the ac servo motor 1 can be reduced if the ac servo motor 1 is located in the central portion of the arm 4 the outer wall of the arm 4 becomes hot as a result of heat generation from the ac servo motor 1 thus causing a burn on an operator in contrast according to the preferred embodiment as the ac servo motor 1 is located at a position distant from a sewing position of the operator there is no possibility of such a burn as the ac servo motor 1 is located not in the central portion of the ar 4 but in the end portion of the arm 4 the operation of assembly and disassembly can be easily carried out to thereby make the maintenance speedy the timing pulley 26 is not directly connected to the output shaft 25 of the ac servo motor 1 but is fixed to the coupling 29 accordingly when separating the output shaft 25 of the ac servo motor 1 from the coupling 29 for the purpose of maintenance or the like it is unnecessary to remove the timing pulley 26 and the timing belt 33 accordingly in mounting the ac servo motor 1 into the sewing machine 3 readjustment of the timing between the main shaft 12 and the bottom shaft 31 is not necessary that is readjustment of the operation timing between the needle bar 30 and the feed dog 49 is not necessary thereby greatly improving an.