We investigate a 4-state ferromagnetic Potts model with a special type of geometrical frustration on a three dimensional diamond lattice by means of Wang-Landau Monte Carlo simulation motivated by a peculiar structural phase transition found in β-pyrochlore oxide KOs_2O_6. We find that this model undergoes unconventional first-order phase transition; half of the spins in the system order in a two dimensional hexagonal-sheet-like structure, while the remaining half stay disordered. The ordered

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... eets and the disordered sheets stack one after another. We obtain a fairly large residual entropy at T = 0 which originates from the disordered sheets.