A wrench including a stem having a head section at one end of the stem and a polygonal socket formed in the head section for fitting onto a screwed member. A resilient ring is disposed at the head section and pivotally connected with a controlling switch pivotally disposed at the head section. By means of operating the controlling switch, the resilient ring is driven to expand or close. In the closed position, the resilient ring obstructs at least one inner angle of the socket to prevent the screwed member from detaching out of the socket and clamp the screwed member in the socket. In the expanded position, the resilient ring frees the inner angle, permitting the screwed member to freely pass through the socket.