PURPOSE: To obtain a mechanism for preventing a casing cover from opening wherein the cover does not drop out, when a cover shutting latch for opening and closing the side surface of a cabinet is forgotten to be fastened, or when the latch comes off.

CONSTITUTION: The prior art hook and latch for shutting a cover are installed. A second hook 8 whose one end is formed in a hooked type is fixed to the lower part of a cover 5. A latch 9 constituted of a spring plate whose one end is formed in a hooked type is fixed to a casing 1, in order to engage with the part of the second hook 8 which part is formed in the hooked type, in the state that the cover 6 is closed, and to prevent the cover 5 from opening. Protective plates are installed on the side surface of the second hook 8 and on that of the engaging latch 9.