A protective casing structure of an electronic apparatus capable of keeping the apparatus compact and light is provided. A display-side block includes a display module and a display casing. The display casing is provided with frame edges at a periphery thereof, with projections formed on the frame edges. Grooves engaging with the projections are formed in a system casing. When the display-side block is closed, the projections engage with the grooves to increase rigidity against pressing from the outside in the closed state. The casing structure thereby increases the protective capability of the display module while keeping the apparatus compact and light.