After AlH3 is synthesized, ball milling is performed under a condition in which a force of 2 G to 20 G (G represents the acceleration of gravity) is applied, to thereby provide AlH3 having an X-ray diffraction pattern in the form of a halo pattern. That is, for example, nanostructured AlH3 is provided, in which a grain boundary phase intervenes in a matrix phase, a side length t2 of the matrix phase is not more than 20 nm, and a width w2 of the grain boundary phase is not more than 10 nm. Alternatively, amorphous AlH3 may be provided. Further, hydrogen is released from AlH3 on which ball milling has been completed, and then the hydrogen is absorbed to induce a change into AlHx (provided that 0