Provided are a method of forming pores in a graphitic carbon nanomaterial and a method of introducing an oxygen-containing group into the pores, in which the rate of pore formation in the wall of a graphitic carbon nanomaterial can be heightened and the amount of the oxygen-containing group, especially the carboxyl group to be introduced can be significantly increased. The method of forming pores in a graphitic carbon nanomaterial of the invention is characterized by forming pores in the wall of a graphitic carbon nanomaterial in the presence of an oxidizing agent while the nanomaterial is irradiated with a light from a light source including a light having a wavelength at which the oxidizing agent is activated.
