A working medium includes a first amine compound and a second amine compound. The first amine compound is a heterocyclic tertiary amine compound including a carbon atom, a nitrogen atom and a hydrogen atom, and in which a ratio (C/N ratio) of a carbon atom number to a nitrogen atom number included in one molecule is from 7 to 9. The second amine compound is a heterocyclic tertiary amine compound including a carbon atom, a nitrogen atom and a hydrogen atom. and in which a ratio (C/N ratio) of a carbon atom number to a nitrogen atom number included in one molecule is in a range of 5 or more to less than 7.