To provide an absorption promotor that is capable of improving the efficiency in absorption of a pharmaceutical drug and that is highly stable, an absorption promotor includes an oxa acid having a structure represented by the formula (1) below,where R1 represents a linear or branched hydrocarbon group having 1 to 40 carbon atoms, one or more but not all carbon atoms of which hydrocarbon group have optionally been substituted by an oxygen atom, a nitrogen atom, or a sulfur atom, the hydrocarbon group being saturated or unsaturated； m represents a real number of 0 to 50； n represents an integer of 1 to 5； and - C(=O)-X represents a functional group capable of chemical reaction. (Fig. 1)
