A method of controlling a network switching device is provided. Perform a mode-process including setting the determination-operation mode of a target switching block to a determination-operation mode that is different from the determination-operation mode of the first switching block while the first switching block is handling the switching-determination process. The target switching block is one switching block selected from the second switching blocks. Then, perform a switchover process including starting the switching-determination process using the target switching block instead of the first switching block, after completion of the process of setting the determination-operation mode of the target switching block.