A switch device includes: a switch unit configured to relay an Ethernet frame between a plurality of function units installed in a vehicle; a monitoring unit configured to monitor predetermined data in the Ethernet frame; and a specification unit configured to specify a protocol being used by a target function unit which is a function unit serving as a transmission source of the Ethernet frame, based on a monitoring result of the monitoring unit. The monitoring unit performs an operation monitoring process of selectively monitoring an operation of the target function unit according to the target protocol that is the protocol specified by the specification unit.