PURPOSE: To enhance mechanical strength and to simplify constitution by forming a rib for reinforcement and a rib for constituting a speaker box in such a manner that these ribs are commonly used.

CONSTITUTION: The part of an upper wall surface 12b of an upper casing 12 is integrally projectingly provided with the rib 23 in a part corresponding to a groove 22 on the inside surface thereof. This rib 23 faces the direction orthogonal with the upper wall surface 12b of the upper casing 12. The rib 23 is commonly used as the rib for reinforcement to prevent the deflection of the upper casing 12 and the rib for the speaker box 15. The bottom surface 13b of the lower casing 13 is integrally projectingly provided with the rib 24 corresponding to the rib 23 of the upper casing 12. The rib 24 faces the direction orthogonal with the horizontal base wall 13b of the lower casing 13. The rib 24 is commonly used as the rib for reinforcement to prevent the deflection of the lower casing 13 and the rib for the speaker box 15.