According to a chain saw braking system, engagement between a front end portion of a belt and a main body by an engagement member is released at the time of application of an impact. When a front end portion is pulled by the spring force of a compression coil spring, an elongate hole for protrusion is engaged with a protrusion to restrict a rear end portion from being moved in the same direction as the front end portion. Thus, it is possible to strongly tighten a drum by the belt to thereby reliably stop the movement of a chain. With this arrangement, the chain can be prevented from being damaged when a chain saw is accidentally dropped onto the ground due to an impact applied thereto.
