A head diaphragm adjustor for adjusting tension applied to upper and lower head diaphragms of a drum. A first bolt adjusts the tension applied to the upper head diaphragm. A second bolt adjusts the tension applied to the lower head diaphragm. A first meshed portion and a second meshed portion are respectively meshed with the first bolt and the second bolt and fixed to the shell. The upper portion of a rod is inserted in the first bolt in a manner independently rotatable relative to the first bolt. The lower portion of the rod is connected to the second bolt in a manner slidable relative to the second bolt and rotated integrally with the second bolt. The rod includes a cylindrical portion having a diameter greater than that of the upper portion of the rod. The first bolt abuts against the cylindrical portion.