There is provided a method that can realize the treatment of an amine compound, a deterioration product thereof and the like in a simple and a low-cost manner. The method includes treating a carbon dioxide absorbing agent, comprising a step in which at least one carbon dioxide absorbing agent represented by OH—R2—NR3—R4—OH or H—NR6—R7—NR8—H into contact with a compound containing two or more isocyanate groups in its molecule to produce a solid containing a urethane bond or a urea bond. In the above formula, R2, R3, R4, R6, R7, and R8 each independently represent CvHwOxNy wherein v, w, x, and y are 0≤v≤10, 0≤w≤22, 0≤x≤3, and 0≤y≤3, respectively.