To provide a belt tension adjustment device by which imbalance of tension resulting from friction between pulleys and a belt can be cancelled.

When an operator presses a sharing switch 68 arranged in the vicinity of an upper portion of a grip part 41 and a start switch 60 arranged in the vicinity of an upper portion of a grip part 42, high pressure air is supplied to a cylinder unit 44 which starts compressing. A compressor pulley 26, which is operatively associated with this operation, is pushed in a direction separating from a crank pulley 22. Then, the compressor pulley 26 rotates about a bolt 28 to apply a tensile force, which is equal to or larger than a set value, to a belt 27. After the lapse of a predetermined time, by a command from a control unit 80, an air circuit of a high pressure operation unit 45 is switched to an air circuit of a low pressure operation unit 46. Then, a force pushing the crank pulley 22 is reduced, so that the belt 27 shrinks due to a repulsive force of its own and the crank pulley 22 is slightly returned in a direction opposite to the above-mentioned direction. The crank pulley 22 is stopped rotating at a position where a regular tension is applied to the belt 27.