A tension-adjusting mechanism for adjusting tension on an endless loop member associated with a driven transmission gear is operable to adjust the tension of an endless loop member on a vehicle. The tension-adjusting mechanism is made compact to reduce weight of the vehicle. In particular, the rear end of an arm of a frame rear fork includes an elongated adjustment slot, and one end of an axle, passed through the adjustment slot, is supported by a slider. The rear end of the fork is provided with a projecting portion on a front side relative to the slider, and a tension-adjusting bolt is threadably engaged with a threaded hole provided in the projecting portion. The shank portion of the tension-adjusting bolt is provided with an anti-turn recessed portion, and a click ball is engaged with and disengaged from the anti-turn recessed portion while being spring-biased to engagement therewith, whereby the click ball, in cooperation with the anti-turn recessed portion, serves as an anti-turn section for the tension-adjusting bolt.