To prevent a subscriber-side terminal device from being affected by streamed noises by imparting a function interrupting the streamed noises flowing into the subscriber-side terminal device from a system-side transmission line to a protector used in a high-frequency transmission system.

A primary winding L1 for an insulating transformer T1 is connected to the central electrode 2a and outside electrode 2b of an input terminal 2, the central electrode 4a and outside electrode 4b of an output terminal 4 are connected to a secondary winding L2 for the insulating transformer T1 respectively through windings La, Lb for a noise eliminating transformer T2, and the outside electrode 4b of the output terminal 4 is connected to the outside electrode 2b of the input terminal 2 through a capacitor C1 for interrupting a DC signal. Accordingly, the common mode component of streamed noises inputted from the outside electrode 2b of the input terminal 2 is cancelled together with a normal mode component inputted through the central electrode 2a in the primary winding L1 for the insulating transformer T1. A part of the common mode component flows to the secondary winding L2 side through the insulating transformer T1 and the capacitor C1, but it is eliminated by the noise eliminating transformer T2.