PURPOSE: To easily mount the base on the housing of an equipment by fitting hooks of fixed arms which are provided at both the front and rear ends of the base in holes formed in the housing while a support member on the reverse surface of the base is made to abut on the back upper end of the equipment housing.

CONSTITUTION: The support member 6 which abuts on the back upper end of the equipment housing 1 is arranged on the reverse surface of the bottom plate 9 of the extension installation base 5 and the fixed arms 7 and 8 fitted with the hooks 7a and 8a fitted in the holes formed in the housing 1 are provided at both the front and rear ends of the extension installation base 5. The rear end 10 of the bottom plate 9 is folded upward so that a mounted equipment does not fall. The support member 6 not only determines the mount position, but also increases the strength of the extension installation base 5 itself. Consequently, even when the external equipment such as a printer is mounted, it can be supported with the sufficient strength.