A communication line isolator (10, 10B) and a protection target device (40, 40B) wherein the communication line isolator is adapted to block an abnormal voltage entering from a communication line to protect the protection target device (40, 40B), wherein the communication line isolator (10, 10B) is configured with an isolation transformer including a primary winding and a secondary winding connected to the protection target device (40, 40B) that is adapted to perform communication with a communication line, wherein: the protection target device (40, 40B) includes a pulse transformer, an inner capacitance (Ci) and an inner circuit; the protection target device (40, 40B) is adapted to transmit/receive a pulse signal to/from the communication line via the pulse transformer and to perform the communication, wherein the pulse transformer includes: a primary winding connected to a terminal of the secondary winding of the isolation transformer, and a secondary winding, characterised by a medium tap in the primary winding of the pulse transformer, wherein the medium tap is grounded through the inner capacitance (Ci) of the protection target device (40, 40B); wherein the isolation transformer comprises a parasitic capacitance (Cs) caused between the primary winding and the secondary winding in the isolation transformer, wherein the parasitic capacitance (Cs) is connected in series with the inner capacitance (Ci), and wherein a voltage drop of the abnormal voltage is divided as a voltage drop over the parasitic capacitance (Cs) and a voltage drop over the inner capacitance (Ci), and, wherein the primary winding of the isolation transformer is connected to the communication line, the secondary winding of the pulse transformer is connected to an inner circuit terminal of the protection target device (40, 40B), and a capacitance value of the parasitic capacitance (Cs) is set such that the voltage drop over the inner capacitance (Ci) is equal to or below a predetermined value, wherein the predetermined value is a withstand voltage of the protection target device (40, 40B), and wherein the abnormal voltage is equal to or below a withstand voltage of the communication line isolator (10, 10B). the protection target device (40, 40B) includes a pulse transformer, an inner capacitance (Ci) and an inner circuit; the protection target device (40, 40B) is adapted to transmit/receive a pulse signal to/from the communication line via the pulse transformer and to perform the communication, wherein the pulse transformer includes: a primary winding connected to a terminal of the secondary winding of the isolation transformer, and a secondary winding, characterised by a medium tap in the primary winding of the pulse transformer, wherein the medium tap is grounded through the inner capacitance (Ci) of the protection target device (40, 40B); wherein the isolation transformer comprises a parasitic capacitance (Cs) caused between the primary winding and the secondary winding in the isolation transformer, wherein the parasitic capacitance (Cs) is connected in series with the inner capacitance (Ci), and wherein a voltage drop of the abnormal voltage is divided as a voltage drop over the parasitic capacitance (Cs) and a voltage drop over the inner capacitance (Ci), and, wherein the primary winding of the isolation transformer is connected to the communication line, the secondary winding of the pulse transformer is connected to an inner circuit terminal of the protection target device (40, 40B), and a capacitance value of the parasitic capacitance (Cs) is set such that the voltage drop over the inner capacitance (Ci) is equal to or below a predetermined value, wherein the predetermined value is a withstand voltage of the protection target device (40, 40B), and wherein the abnormal voltage is equal to or below a withstand voltage of the communication line isolator (10, 10B). a primary winding connected to a terminal of the secondary winding of the isolation transformer, and a secondary winding, characterised by a medium tap in the primary winding of the pulse transformer, wherein the medium tap is grounded through the inner capacitance (Ci) of the protection target device (40, 40B); wherein the isolation transformer comprises a parasitic capacitance (Cs) caused between the primary winding and the secondary winding in the isolation transformer, wherein the parasitic capacitance (Cs) is connected in series with the inner capacitance (Ci), and wherein a voltage drop of the abnormal voltage is divided as a voltage drop over the parasitic capacitance (Cs) and a voltage drop over the inner capacitance (Ci), and, wherein the primary winding of the isolation transformer is connected to the communication line, the secondary winding of the pulse transformer is connected to an inner circuit terminal of the protection target device (40, 40B), and a capacitance value of the parasitic capacitance (Cs) is set such that the voltage drop over the inner capacitance (Ci) is equal to or below a predetermined value, wherein the predetermined value is a withstand voltage of the protection target device (40, 40B), and wherein the abnormal voltage is equal to or below a withstand voltage of the communication line isolator (10, 10B).