The object of the invention is to provide an adsorbent that can adsorb ammonia with no large volume change between absorption and desorption, that has a high ammonia and/or ammonium ion adsorption capacity, and that can have an additional function by gaining proper control of composition, etc. The invention makes it possible to provide an adsorbent that absorbs ammonia and/or ammonium ions through the use of a metal cyanocomplex as an ammonia adsorbent, experiences no or little volume change, exhibits high enough capacity for adsorbing ammonia and/or ammonium ions, and has a function of decomposing ammonia as well as a function of varying optical responses before and after adsorption, etc.