The present invention provides a carbon dioxide absorber capable of efficiently and stably removing carbon dioxide in a gas or solution. This carbon dioxide absorber contains an amine compound, a weakly acidic compound and water, the pKb value of the amine compound in an aqueous solution at 30° C. is 4.0 to 7.0, the pKa value of the weakly acidic compound in an aqueous solution at 30° C. is 7.0 to 10.0, and the weakly acidic compound is present in an amount within the range of 0.01 equivalents to 1.50 equivalents with respect to amino groups of the amine compound.