To provide a common mode radiation inhibit circuit for a differential transmission line which can inhibit quadratic common mode radiation by ensuring a continuity of grand level at a point of change of a signal grand, including a differential transmission line which is constituted of a pair of signal wiring, and transmits differential signals, signal grounds which are arranged along the differential transmission line, and each have a change point on its way, a transformer, and a reference ground, wherein a primary side of the transformer is connected between an electric midpoint of the pair of signal wiring, and the signal ground at one side of the front and back of the change point of the signal grounds, wherein the secondary side of the transformer is connected between the electric midpoint of the pair of signal wiring and the signal ground at another side of the front and back of the change point, and wherein the reference ground is connected to a point which becomes a neutral sheet of the primary side of the transformer and a point which becomes a neutral sheet of the secondary side of the transformer.