A flat wood diaphragm 105×105 mm in size for a low to middle range loudspeaker was developed using the wood for soundboards of musical instruments. It is necessary forthe wood diaphragm to be humidity-proof and isotropic. For the former, coating with varnish, which was the same as that used for the soundboards, and acetylation, which was essential in the conversion of hydrophile property, were carried out. The humidityproof performance of these treatments was tested, and each treatment showed the characteristic effect. For the, latter, rectangular wood bars for reinforcing radial (R) direction, which was lower in strength, were glued to both edges of the square wood board, one side of which was parallel to longitudinal (L) direction, in L direction. It was found that the wood board with the bars, wood diaphragm, had a circle node at quasi-isotropy, and it was clarified that the circle node was produced by the degeneration of both vibrations in each direction of L and R. Loudspeakers with the developed wood diaphragm showed acceptable performance.