To provide a case structure which is capable of improving a signal generator in EMI characteristic and superior in maintainability.

A board 4 is housed in an inner shield case 5, and high-frequency electromagnetic waves generated by the board 4 are doubly shielded by the inner shield case 5 and an outer shield case 2. The openings 2a and 5a, which are each provided to the cases 2 and 5 are at the same level and can be stopped up with only a case lid 2b at the same time. A support plate 21 is provided to the front plate 2f of the outer shield case 2, and coaxial connectors 20 are fixed to the support plate 21. An attenuator 11 is mounted on the front plate 2f. When the front panel 30 is dismounted, the case structure can be improved in degrees of freedom, and the attenuator 11, and an LCD 34, a switch and others provided on a front panel 30 can be maintained easily. A conductive leaf spring 23 is provided between the coaxial connectors 20 and the front panel 30, and the outer conductors of the coaxial connectors 20 are grounded to shield leakage electromagnetic waves.