[en] A new LDA+U band calculation is performed on the Kondo insulator material YbB₁₂ and an energy gap of about 0.001 Ryd is obtained. Based on this, a simple tight-binding model with 5dε and 4f Γ₈ orbitals on Yb atoms and the nearest neighbor σ-bonds between them is constructed with a good agreement to the above the LDA+U calculation near the gap. The density of states is also calculated and the shape is found to be very asymmetric with respect to the gap. A formation mechanism of the gap is clarified for the first time in a realistic situation with the orbital degeneracies in both conduction bands and the f states. This model can be a useful starting point for incorporating the strong correlation effect, and for understanding all the thermal, thermoelectric, transport and magnetic properties of YbB₁₂. (author)