A common mode filter, comprising: a lumped-constant differential delay line formed by passive series elements and passive parallel elements, in a ladder-shaped differential four terminal network composed of the passive series elements arranged in series in each differential line (1, 3) and the passive parallel elements arranged in parallel between the differential lines (1, 3), the passive series elements being first inductors (Lo), the passive parallel elements being capacitors (Co, C 0 /2, C 0 /4), the capacitors (C 0, C 0 /2, Co/4) including: two capacitors (C 0, C 0 /2) having equal values and connected in series; and a further inductor (L1, L2, L3, L4) for attenuating a common mode noise connected between a connection point (T1, T2, T3, T4) of the capacitors (Co, C 0 /2) connected in series and a ground potential, so that an attenuation pole for attenuating the common mode noise is formed together with the capacitors (Co, C 0 /2), thereby forming a differential delay element in which the first inductors (L 0) that form the passive series elements are equally divided into two, the equally divided inductors are connected in series and so that mutual induction occurs between them, and connection points of the two equally divided inductors (L 0 /2) are connected by a series circuit of the capacitors. a lumped-constant differential delay line formed by passive series elements and passive parallel elements, in a ladder-shaped differential four terminal network composed of the passive series elements arranged in series in each differential line (1, 3) and the passive parallel elements arranged in parallel between the differential lines (1, 3), the passive series elements being first inductors (Lo), the passive parallel elements being capacitors (Co, C 0 /2, C 0 /4), the capacitors (C 0, C 0 /2, Co/4) including: two capacitors (C 0, C 0 /2) having equal values and connected in series; and a further inductor (L1, L2, L3, L4) for attenuating a common mode noise connected between a connection point (T1, T2, T3, T4) of the capacitors (Co, C 0 /2) connected in series and a ground potential, so that an attenuation pole for attenuating the common mode noise is formed together with the capacitors (Co, C 0 /2), thereby forming a differential delay element in which the first inductors (L 0) that form the passive series elements are equally divided into two, the equally divided inductors are connected in series and so that mutual induction occurs between them, and connection points of the two equally divided inductors (L 0 /2) are connected by a series circuit of the capacitors. two capacitors (C 0, C 0 /2) having equal values and connected in series; and a further inductor (L1, L2, L3, L4) for attenuating a common mode noise connected between a connection point (T1, T2, T3, T4) of the capacitors (Co, C 0 /2) connected in series and a ground potential, so that an attenuation pole for attenuating the common mode noise is formed together with the capacitors (Co, C 0 /2), thereby forming a differential delay element in which the first inductors (L 0) that form the passive series elements are equally divided into two, the equally divided inductors are connected in series and so that mutual induction occurs between them, and connection points of the two equally divided inductors (L 0 /2) are connected by a series circuit of the capacitors.