To provide a tension adjusting device of a simple structure.

This device includes a side frame 3 including a pair of left and right side frame structure members 8 opposing at a designated interval, a fixing member 15 bridging over and fixed on the side frame structure members 8 of the side frame 3 and including a bolt insertion through hole 14 having axis directed to a front and after direction, an adjusting bolt 20 fitted in the bolt insertion through hole 14, a front and rear pair of adjusting nuts 21 positioned in a front and a rear side of the fixed member 15 and threadedly fitted on the adjusting bolt 20, and a wheel 5 having an endless carrying belt 6 rotatably provided on a bearing 22 provided on the adjusting bolt 20 with the axis directed to a left and right direction wound around.