To easily and quickly adjust reference tension applied to the upper thread by spring force of a thread tension spring through a pair of thread tension discs.

A thread tension guide is provided with a second adjusting member 34 for rotating a first adjusting member 33 receiving one end of the thread tension spring 28, and a through-hole 24c for inserting an end of a hexagon wrench T threrethrough, so that when tension at a reference position of a thread tension dial 29 is adjusted, an operator is required only to remove a face plate covering a head portion 4 of an electronic sewing machine M, to insert the end of the hexagon wrench T into the inside through the through-hole 24c formed in a thread guide body 24, and to rotate the hexagon wrench by a desired rotation angle in the state that the hexagon wrench is engaged to the engaging recess portion 34d while being guided by the guide portion 34e of the second adjustment member 34; thus the first adjustment member 33 is rotated by the rotation of the second adjustment member 34, and the tension applied to the upper thread is adjusted.