Sewing machine

A sewing machine comprising:

• an upper shaft that drives a needle and a balance;
• a lower shaft that drives a hook and a feed dog;
• an upper and lower shaft belt that connects the upper shaft and the lower shaft and synchronizes rotation of the upper shaft with rotation of the lower shaft;
• and
• an idler unit that includes an idler that comes into contact with the upper and lower shaft belt and pushes the idler into the upper and lower shaft belt to change a tension side belt length,
• wherein the idler unit includes
• a transmission mechanism connected to the upper shaft or the lower shaft and
• a rotating body mechanism provided with the idler and having a rotating body that rotates in synchronization with the upper shaft or the lower shaft by the transmission mechanism, and wherein
• a pushing amount of the idler into the upper and lower shaft belt is changed according to rotation of the rotating body.