Sewing machine

A sewing machine comprising:

• an upper shaft rotatable so as to drive a needle bar;
• a lower shaft rotatable so as to drive a feed dog;
• an upper-shaft pulley provided for the upper shaft;
• a lower-shaft pulley provided for the lower shaft;
• a belt linking the upper-shaft pulley with the lower-shaft pulley to synchronize respective rotations;
• a belt adjusting mechanism contacting the belt, and changing a belt length at a tensioned side of the belt where the belt is drawn in and a belt length at a loosened side of the belt where the belt is drawn in;
• and
• a tension adjusting idler applying force to the belt in accordance with a change in the belt length,
• wherein the belt adjusting mechanism comprises:
• two idlers contacting to the belt at the tensioned side and the loosened side respectively;
• an arm supporting two idlers;
• and
• a motor swinging the arm,
• wherein when the adjusting mechanism changes the belt length at the tensioned side so as to elongate the belt length, a feeding phase of the lower shaft is advanced relative to a thread take-up lever linked with the upper shaft, and when the adjusting mechanism changes the belt length at the loosened side so as to elongate the belt length, a feeding phase of the lower shaft is retarded relative to a thread take-up lever linked with the upper shaft.