A lattice model of protein is studied by a Monte Carlo simulation method. The native conformation of the lattice protein molecule is stabilized by specific long-range and short-ranged interactions. By comparing results of simulation for different relative weights of the long- and short-range interactions, it is concluded that the specific long-range interactions are essential for highly cooperative stabilization of the native conformation and that the short-range interactions accelerate the folding and unfolding transitions.