front end of the hook 76 further slides forward on the portion of the lower surface of the tension bar 56 between the front end face 78 of the tension bar 68 and the inclined forward wall face 74 of the recessed receiving portion 73 thereof and finally rides over the front end face 78 when the pulley holder 60 is released in this state the guide shoe 62 is moved toward the tension bar 56 by its spring force and the hook 76 engages the forward wall face 78 in this case since the tension bar 56 is kept pushed in the supporting member 54 the grinding belt 4 is kept loosened from the pulleys 58 and 59 as shown in fig9 in the state where the grinding belt 4 is loosened it can be removed from the pulleys 58 and 59 or replaced with a new one or a new grinding belt 4 is mounted on the pulleys 58 and 59 after attachment and replacement of the grinding belt 4 the hook disengaging pieces 77 are held by the operator 39 s fingers and pulled in the direction in which they are separated from the front portion 68 of the tension bar 56 the hook 76 is disengaged from the front portion 78 and the tension bar 56 is moved forward to tighten the grinding belt 4 next the hook disengaging pieces 77 are.