[Object] To provide a method of manufacturing a dielectric device and an ashing method that are capable of suppressing the occurrence of resist residue.[Solving Means] In the ashing method, a base material having a surface etched by a plasma of chlorine gas or fluorocarbon gas via a resist mask (6) formed of an organic material is disposed in a chamber, bombardment treatment is performed on the resist mask (6) by using oxygen ions in the chamber, and the resist mask is removed by using oxygen radicals in the chamber. According to the ashing method described above, etching reactants adhering to the surface of the resist mask are physically removed by the bombardment treatment using oxygen ions. Thus, it is possible to suppress the occurrence of resist residue due to the etching reactants and efficiently remove the resist mask from the surface of the base material.