Provided is a windmill device including a wind tunnel member with a wind tunnel passage and a windmill held rotatably inside the wind tunnel passage of the wind tunnel member. The windmill includes a hub serving as a rotation center, plurality of spokes extending radially therefrom, an approximately annular limb connecting outer ends of the plurality of spokes, and plurality of sailcloth pieces, each sailcloth piece being approximately triangular in shape, one side of each sailcloth piece left as a free end 24a not supported by any of the spokes and the limb. A ratio S2/S1 is 0.9 or more. S1 is a cross-sectional area of the wind tunnel passage. S2 is a wind catching area of the plurality of sailcloth pieces.