A method of processing a cavity of a core substrate is disclosed. The method of processing a cavity of a core substrate in accordance with an embodiment of the present invention can include: forming a first processing area on one surface of a core substrate, the first processing area being demarcated by a circuit pattern; forming a second processing area on the other surface of the core substrate, the second processing area being demarcated by a circuit pattern; and processing a cavity by removing the entire first processing area from the one surface of the core substrate.