To easily adjust the tension without stopping an engine by providing an adjusting member which engages with a tension pulley fitting bracket and pierces through a mounting table, screw-engaging a nut with a screw part provided on a part projected from the mounting table, and engaging or rotating the nut with the mounting table.

A mounting table 90 is provided with a horizontal hole 13 which is communicated with a groove 14 provided from the right. An adjusting member 15 having an L-shaped bent part 15a at one end and a screw part 15b at the other end is stored in the groove 14, the screw part 15b is pierced through the hole 13, and the screw part 15b projected from the mounting table 90 is screw-engaged with a nut 16. A bent part 15a is inserted in a hole 17 provided in a tension pulley fitting bracket 98. The fitting bracket 98 is turned on a shaft 2 with respect to the mounting table 90 and firmly fixed by fastening a bolt 12 or the like. The bolt 12 is loosened to turn the nut 16, thereby moving the adjusting member 15 in the lateral direction to adjust the tension of a fan belt 1.