The present invention provides a method for stabilizing an oral solid formulation containing a benzimidazole-based compound or a physiologically acceptable salt thereof. That is, it provides a method for stabilizing a benzimidazole-based compound or a physiologically acceptable salt thereof, comprising incorporating 1) a crospovidone or a crospovidone and 2) sodium hydroxide and/or potassium hydroxide to a benzimidazole-based compound or a physiologically acceptable salt thereof.