To provide a portable timepiece whose vent valve construction is simple, in which the vent valve is not easily erroneously opened, and in which water is not easily allowed to enter the timepiece exterior assembly with the completion of venting.A wristwatch is equipped with a vent valve mounted to a timepiece exterior assembly. The vent valve is equipped with a pipe, a valve body, a retaining member, and packing formed of a seal material capable of elastic deformation. The timepiece exterior assembly is equipped with the pipe establishing communication between the interior and exterior thereof. The pipe has a valve seat surface, a stopper portion, and a male screw portion. The valve body is equipped with a valve body shaft portion inserted into the pipe, and a valve body head integral with this shaft portion. The valve body head is provided with a female screw portion threadedly engaged with the male screw portion and a plurality of vent holes, with the valve body being axially movable through a change in the mesh engagement of both screw portions. There is mounted to the valve body shaft portion the retaining member moving toward and away from the stopper portion from the interior side of the timepiece exterior assembly. The waterproof packing moving toward and away from the valve seat surface as the valve body moves is mounted to the valve body shaft portion.