Casing structure for a mixer apparatus includes a box-shaped casing that comprises an upper surface section, a bottom surface section and a side surface section. The upper surface section, having a generally obtuse-angled “V” shape, includes: a flat operation panel having operating members; a boundary section located adjacent to the rear end edge of the operation panel; and a flat, inclined display panel extending rearwardly and upwardly from the boundary section at a greater inclination angle than the operation panel. Within the casing, a partition member is provided on the bottom surface section to extend vertically upward to the boundary section, and it has openings to permit communication between front and rear accommodating sections. The partition member supports the boundary section from below to reinforce the upper surface section.