PURPOSE: To provide an inexpensive thread tension detecting device for sewing machine capable of precisely detecting a thread tension at sewing start or even when the rotating speed of a sewing machine is changed.

CONSTITUTION: Thead tension detecting devices 17, 20 for sewing machine arranged in the thread guide from a thread supplying source 12 to a thread consuming part 19 to detect the thread tension by the contact with the thread and having a piezoelectric element at least in a part are intended, and they have an oscillation output means for voltage or current-driving the piezoelectric element by the resonance frequency or anti-resonance frequency of the piezoelectric element, and a voltage waveform detecting means for detecting the voltage waveform given to the piezoelectric element by the oscillation output means. Further, the devices have a current waveform detecting means for detecting the current waveform given to the piezoelectric element by the oscillation output means, and a tension detecting means for detecting the thread tension on the basis of the voltage waveform and the current waveform.