To provide a belt tensioner adjusting a belt into a highly precise and reproducible tensioning state.

This belt tensioner 11 shifts a pulley 26 and tensions the belt 30 hung around the pulley to a prescribed tensioned state. This device is provided with a rod 34 connected to the pulley, a member 42 movable along the rod with its connected to the rod, and a spring 46 applying a reaction to the member 42, when the member is moved in a first direction along the rod. This device 11 is also provided with a spacer 44 engaged with the member 42 so as to limit the advance of the member in the first direction.