A protective device is applicable to a casing provided with a first coupling portion and configured for use with an outdoor communication apparatus. The protective device includes a body and a shielding portion. The body is configured to shield the front of the casing and provided with a second coupling portion coupled to the first coupling portion. The shielding portion is laterally provided to the side of the body for shielding the casing sideways. Hence, the shielding portion of the protective device shields the casing from direct irradiation of sunlight, prevents the outdoor communication apparatus from being overheated, and protects the outdoor communication apparatus against rainwater and dirt. A casing configured for use with an outdoor communication apparatus and equipped with the protective device is further provided.