VLAN is in the method of using a network flexibly in LAN. This VLAN is logical LAN built on the network consisted of some switches (Switch Network), and many switch networks are used for this use. In a switch network, existence of apparatus cannot be checked from IP layer. Therefore, it is difficult to specify a problem at the network troubleshooting. This research describes the technique of presuming the number of the switch apparatus in a unknown link and reports the result of an evaluation experiment of the proposal technique.