PURPOSE: To prevent an unidentified person from approaching a cylinder, and to make it difficult for the person to enter a house, etc., by providing a housing which is located opposite to the cylinder and provided with a bore adaptable to the size of a key, and a protecting means for the cylinder which is located between the housing and the cylinder, and connected to an alarm actuating means.

CONSTITUTION: A bore 2 is covered by a plate 4, and the plate 4 is connected to a shaft 5 on which a spring 6 is arranged. Then, an arm 7 is fixed and connected to the plate 4 so as to be served for a microswitch 8. The arm 7 is moved along the microswitch 8 and the microswitch 8 operates an alarm actuating means. A sensor 9 is directly connected to the alarm actuating means, and the alarm actuating means is operated by the sensor 9 as soon as someone attacks a housing 1.