The invention provides a thread tension device of a sewing machine controlling a wide range of thread tension while dispensing with a resistor with a large allowable current value. This thread tension device is provided with: a thread tension appliance (93) generating a tensile force to a sewing thread of a sewing machine; an actuator (19) becoming a drive source of the tensile force generation; a control means (50) outputting a control signal according to a target tensile force; and a driving circuit (20) driving the actuator by energizing a drive current according to the control signal. The drive circuit is provided with: detecting means (44, 45) connected to the actuator, detecting the drive current and capable of changing over a detection resistance value; and a resistor changeover means (35) changing over the detection resistance value by the target tensile force. When the target tensile force is larger than the prescribed value, the drive circuit detects the drive current at a small detection resistance value and controls the drive current. |