PURPOSE: To make the turning-on voltage of an overvoltage protective device lower than that of (n) pieces of VBO-free thyristors connected in series within a prescribed range by constituting the impedance circuit of the device of two kinds of impedance circuits having different impedances.

CONSTITUTION: When the impedances Z1, Z2, and Z3 of the voltage dividing resistors of the title overvoltage protective device composed of three voltage dividing resistors connected in series are selected so that the impedances can become V1:V2:V3=1.5:1:1, the voltage applied across a VBO-free thyristor 11-1 becomes higher than those V2 and V3 respectively applied across VBO-free thyristors 11-2 and 11-3 and the ratio among them becomes V1:V2:V3=1.5:1:1. In case the turning-on voltage of the thyristor 11 is VVBO, the voltage applied across the three serial circuits becomes (1+(1/1.5)+(1/1.5))VVBO=2.33VBVO and the thyristors 11-2 and 11-3 are successively turned on when the thyristor 11-1 is turned on, a voltage, (2.33/2)VVBO=1.17, is equally applied across the thyristors 11-2 and 11-3.