To provide a belt conveyor device as improvement of a conventional one of expansion type, in which it is required to install head belt pulleys at two stages, install an intermediate belt pulley for changing the direction of the belt running, and provide a long belt to result in a complicated constitution.

The belt conveyor device is configured so that the center C of the side belt pulleys 4 and 5 where the insides of the two ends of the belt 3 in a ring shape are set over is moved for the same distance (stroke S) in the direction perpendicularly intersecting the axes 4a and 5a and thereby the length of the transport path for a band-shaped material is made adjustable.