To provide a belt installing device fittable to a pulley without damaging a belt; and a belt installing method.

This belt installing device 1 has a guide part 2, a locking part 3 and a movable part 4 composed of a first member 5 and a second member 6. The belt installing device 1 is fixed to the pulley 21 by fitting a groove of the locking part 3 to a flange 21a of the pulley 21 being a pulley to be installed, and is placed on the second member 6 by extending the belt 20. The movable part 4 is rotated in the belt fitting surface 21b direction via a hinge part 7, and the belt 20 is slid to the first member 5 and the guide part 2, and is moved on a substantially straight line of a belt fitting surface 21b. While avoiding the pulley 21 by moving the belt installing device 1 along the outer periphery of the pulley 21, a V rib 20c of the belt 20 is fitted to a pulley groove 21c of the pulley 21.