A processing method according to the present embodiment is a processing method of a fiber containing resin in which fibers are contained in a matrix resin. The processing method includes: a step of thermal decomposition of the matrix resin in the fiber containing resin; and a step of stirring a resulting fibers bundle in solvent after the thermal decomposition. At the time of the thermal decomposition, the matrix resin may be carbonized in a dry distillation-carbonization furnace, for example.