A variable ISI transmission channel apparatus inserted in a high-speed transmission channel for the high-speed serial data communication simulates a “live” ISI environment to which the high-speed transmission channel in operation is exposed, in a situation such as a bit error test by means of continuously adjusting an amount of inter-symbol interference (ISI) in the high-speed transmission channel.By allowing an undersurface of a transmission loss generating member (6) (16) (26) such as a dielectric, a magnetic body, and an electric conductor to face, and slide on, a top surface of a conductor strip (2) exposed on a top surface of a plate-shaped dielectric substrate (1), a facing area is continuously increased/decreased. A dielectric loss, a magnetic loss, or a resistance loss increased/decreased in the transmission loss generating member is reflected on a high-frequency signal on the conductor strip (2).