PROBLEM TO BE SOLVED: To provide a foaming agent which can maintain a foaming quantity and a foaming lasting time in a balanced manner even when dissolved in water at about 60°C or more (warm water to hot water).SOLUTION: A foaming agent comprises (A) fumarate and (B) hydrogen carbonate, wherein the content of (A) fumarate is 8-20 wt.% of the foaming agent, and the content of (B) hydrogen carbonate is 0.5-2 pts. wt. relative to (A) fumarate 1 pts. wt. The foaming agent is used while being dissolved in water at 60°C or more.