A shield is adapted be locked to a casing. The casing is formed with an inserting space, an opening, and two engaging holes. The shield includes a shell body, two engaging members and two biasing springs. The shell body is inserted into the inserting space, and shields the opening. The shell body includes a base plate formed with a slot, and two side plates each formed with a through-hole. Each of the engaging members is slidable relative to the base plate, and includes an engaging portion extending through the corresponding through-hole and releasably engageable with the corresponding engaging hole. Each of the biasing springs biases the corresponding engaging member to move in a direction toward the corresponding through-hole.