Provided is an adsorbent having excellent adsorption performance in a medium-humidity range by employing an amorphous aluminum silicate formed from protoimogolite. The amorphous aluminum silicate formed from protoimogolite is synthesized by setting the Si/Al ratio in solution mixing to be of a condition that is higher than synthesizing imogolite or amorphous imogolite, and performing desalting treatment and heating thereto. The obtained amorphous aluminum silicate formed from protoimogolite yields a superior water vapor adsorption performance of 30 wt % or higher at a relative humidity of 60%, and can be used as an adsorbent for desiccant air conditioning.