PURPOSE: To successively control the tension of a needle thread in accord with the movement of a mechanism by providing a needle thread clamping means for clamping the needle thread in a thread guide between a thread take-up lever and a needle thread supply source and interlocking the command value of clamping force applied to the needle thread clamping means with the operation of the thread take-up lever.

CONSTITUTION: The phase of a thread take-up lever 3 is detected as voltage by a potentiometer 9. A phase detecting signal is A/D converted and input to a one-chip microcomputer 31. In a memory of the one-chip microcomputer 31, one cycle of feature data of a clamping force pattern representing a relation between the phase of the thread take-up lever 3 during a sewing operation and an optimum needle thread clamping force is stored. Namely, the current phase of the thread take-up lever 3 is calculated from a voltage value input from the potentiometer 9. The needle thread clamping force corresponding to the phase is obtained from the clamping force pattern. Then, a current value necessary for generating desired needle thread clamping force in an electromagnet constituting a needle thread clamping means 7 is obtained from calculation and output to a servo amplifier 32. In accordance with the input current command, the servo amplifier 32 applies the desired current to a coil 21 of the needle thread clamping means 7.