In a method for generating a chlorine dioxide gas, the chlorine dioxide gas is continuously generated from a gel composition obtained by adding a gelling activator containing a gas generating agent, a gas generation controlling agent containing a carbonate and hydrogen peroxide, a gas generation adjusting agent, and a water-absorbent resin to a chlorite aqueous solution. This provides a method for generating a chlorine dioxide gas, a kit for generating a chlorine dioxide gas, and a gel composition which suppress the initial rapid generation of the chlorine dioxide gas and stably hold the generation of the chlorine dioxide gas for an extremely long time.